Stop supporting EOL versions of Unity Editor

[webbertakken]

As brought up by @EliasVal:

Why are Unity versions that have reached EOL are still having tests ran with? 2018.x, 2019.x and 2020.x have reached their EOL's a long while back and I think that support should be dropped for versions that Unity doesn't support themselves

We should adapt the workflow to no longer build those outdated and end of life versions of the Unity Editor.

[MrGadget1024]

Unity has been pushing updates for 2021.x all year, including 2021.3.45 today, so I'd think you need to keep supporting what Unity is shipping. Per this post from Unity, 2021 support continues through the official full release of Unity 6 (tomorrow?)

[webbertakken]

Technically in that very post it says that support is "expected by users", not "official" as given by Unity Technologies. They merely show leniency because of that expectation.

Generally speaking, for us as an open source project with limited capacity, it doesn't make sense to support longer than Unity's own official support.

That said, I suppose we can delay this issue until the release of Unity 6 LTS is out.

[MrGadget1024]

support is "expected by users", not "official" as given by Unity Technologies

I think publishing 2021.3.45 today counts as official support, at least through today! :D

Edit: We (Mirror Networking) are OSS too...I feel ya. We've been supporting 2019 through latest, but we're probably culling back to 2022 and later end of the calendar year.