[CI] Go back to ubuntu 20.04

[kLabz]

Ubuntu 18.04 is not available anymore on github actions.
I suppose we could rewrite it all using docker images, but would that be worth it?

As far as I understand, this means:

• no more 32bit linux releases; support for which seem to have been dropped at most places
• ubuntu 18 (which EOL is in a month or so) users will need to build from source or use an older release
• anything else in a worse state than last month with ubuntu 18?

cc @Apprentice-Alchemist @tobil4sk

Past work that this pretty much cancels: #529 #533

[tobil4sk]

we could rewrite it all using docker images, but would that be worth it?

Will hashlink have a release with haxe 4.3.0? If not, then I definitely don't think it's worth it especially since it was only a few months ago that 1.13.0 was released, which still has 18.04 support.

We should try to make an effort to not break anything else for ubuntu 18.04 until the EOL date (31st of May). This way users can still build from source, otherwise they can use 1.13 as you said. The binaries can only ever work on one LTS version of ubuntu at a time due to #595 anyway, so really we're just trading an older version for a newer one with more users ¯\(ツ)/¯.

[kLabz]

I don't know but I kinda doubt it since last version is indeed recent and there haven't been much changes related to hl recently