Replace gdx setup references with Gdx-Liftoff

[raeleus]

This PR builds on the work of slebed in PR #197 and addresses the issues detailed in #181. The goal is to remove all references to the old gdx-setup in favor of the new Gdx-Liftoff. In going through these files, I realize that our wiki is showing its age and really deserves a rewrite. I addressed some additional issues I've come across like missing sections, but that is not my focus with this PR. I only wish to discontinue the use of the old setup app which is a continuing burden to support in our community. The switch to Liftoff has already been decided by the libGDX team and we've taken great strides to make this switch happen. Let's make that final, killing stroke now!

Please review the changes and let me know what else needs to be modified. Honestly, I do not want to see this PR linger on esoteric details that could be addressed at another time. I do not have the capacity to rewrite entire articles even though our wiki sorely needs it. Thank you for your help.

[tommyettinger]

Other than one misspelling (which doesn't actually make the word unreadable), this looks fine! Approved.

[crykn]

Great work! I really appreciate you taking charge on this. I just added a few minor comments. Also, the text on _pages/dev/tools.md could be changed to point to mention liftoff instead of gds-setup.

[Frosty-J]

https://libgdx.com/wiki/extensions/third-party-extension-support also mentions gdx-setup, including a broken link to extensions.xml. Unfortunately, it looks like it would need a total rewrite to remain relevant. "Unless you are an advanced AI extension" has aged pretty well, though.

[crykn]

It should probably just be removed. If anyone wants to get new extensions into liftoff, the liftoff repo would be the right place to ask.

[SonicGDX]

I do apologise for not reviewing this before it was merged, I didn't see this until now.

But yeah, this looks good, the only thing I would have suggested is that the JDK compatibility section seems to be more so about the Java language levels rather than the JDK version, (newer JDKs can target lower language levels). But I'll make a new issue for that.