Ubuntu related releases should use the oldest version aviable

[Kastakin]

As stated on the PyInstaller docs is advisable when building on Linux to always build your app on the oldest version of GNU/Linux you mean to support.

I've created a pull request (#1) where I've edited the version of Ubuntu used in both release-ubuntu.yml and release-multi-os.yml to the oldest version available for the GitHub runners

[trappitsch]

Great catch, thanks @Kastakin