Remove: [CI] Linux Legacy support (#11882)

Although created not long ago, you battled to be relevant.
Sadly, GitHub Runners didn't agree with you.
You can't run node20.
And that makes you a broken legacy.
So many potential.
But here we are. Bye Linux Legacy. Thank you for being.

Fix: [CI] wait for all targets to succeeded before uploading to any (#11845)

Otherwise it is possible Steam upload happens while CDN upload
does not, which is a bit awkward.

Change: [CI] upgrade linux-generic to libc 2.28 and introduce linux-legacy (#11792)

linux-legacy is the old variant, which uses libc 2.17, released in 2012.
linux-generic is now upgraded to libc 2.28, released in 2018.

Feature: opt-in survey when exiting a game

On first start-up, the game will ask if you want to participate
in our automated survey. You have to opt-in, and can easily opt-out
(via the Options) at any time.

When opt-in, whenever you exit a game, a JSON blob will be send
to the survey server hosted by OpenTTD. This JSON blob contains
information that gives a global picture of the game just played:
- What settings were used
- How many humans vs AIs
- How long the game has been played
- Basic information about the OS / CPU

All this information is kept very generic, so there is no
chance we send private information to our survey server.
Nothing in the JSON blob could identify you as a person; it
mostly tells about the game played. At any time you can see
what the JSON blob includes, by pressing the "Preview Survey
Results" button in-game.

Change: [Actions] release Windows Store files to CDN

Currently they had a name that the rest of our system cannot
deal with correctly. "cert.pfx" is also not very descriptive from
a system as a whole.

As such, we now name it like any other file, so it can be published
safely to the CDN.

Fix: [Actions] Remove Ubuntu:groovy from release pipeline (#9845)

It no longer exists upstream. Let's focus on LTS only for now.

Change: [Actions] Switch back to primary source for 'gon' in macOS build job

This reverts commit 7a97a33598774faf65576b55e5d1322716859de6.

Fix #9386: compilers failing to compile with LTO by using variants instead of new + unique_ptr

With std::variant all memory can be figured out at compile time, so the compiler needs to keep track of fewer elements. It also saves out a unique_ptr and its memory management, over a slight impact for resolving a setting.

Fix: [Actions] "is-tag" variable is the 5th element, not 6th

The 6th is "is-stable-tag", but it is currently broken in meaning.
Betas and RCs are considered "stable", but final releases are not.
This is the reason it was working for RC1, but not for the final
release.

Codechange: [Actions] Document better why we build our own fluidsynth (#8646)

nielsmh nicely correct us in #8641, pointing out the old comment
is not telling a complete truth. The result is the same, but it
is better to not mislead future-us.

Feature: [Actions / CMake] support for generic linux builds (#8641)

These bundles can be opened on any "modern" Linux machine with
a driver that SDL2 supports.

Machines needs at least glibc 2.15, which was released 10 years ago.
It is build with CentOS 7 as base, and only assumes the following
libraries are available on the system:
- libc
- libdl
- libgcc_s
- libpthread
- librt
- libstdc++

All other libraries the game depends on are bundled together with
the game, so users don't need any library installed to use this
bundle. The downside of course is that this increases the binary
size a bit: 30 MiB of libraries are in this bundle.

RPATH is used to make ld-linux find the folder libraries are
stored in; however, system libraries are always used before these,
in the assumption libraries on the user system are more up-to-date.

Using -DOPTION_PACKAGE_DEPENDENCIES=ON switches on packaging
of libraries in the "lib" folder. This requires CMake 3.16 to
be installed; otherwise it will fail.

Codechange: [Actions] we no longer need xdg-basedir (#8615)

This was already removed from the "ci-build", but not yet from
"release".

Fix: [Actions] circumvent Windows tar warning about read-only files (#8570)

This was already applied on the CI build, but not yet on the
release build.

Fix: [Actions] cmakeBuildType is only used with CMakeListsTxtBasic (#8435)

We use CMakeListsTxtAdvanced, and as such, we have to do this our
self via "-DCMAKE_BUILD_TYPE=RelWithDebInfo". Otherwise we are
producing Debug builds instead of Release builds. Oops.

Add: [Actions] release workflow for both releases and nightlies

This has several ways of being triggered:
- When creating a new release via the GitHub interface. Fully
automated that will produce new binaries, upload them, and it
will even update the website to tell about the new version.
- When triggered in an automated way from OpenTTD/workflows to
start a nightly.
- Manually via the Release workflow, which accepts branches,
Pull Requests and tags to build.

Rerunning a job is safe and should be without issues. Everything
retriggers and updates what-ever might have been broken. In fact,
except for dates, it should produce identical results.

Co-authored-by: Charles Pigott <charlespigott@googlemail.com>