Obsolete Ubuntu Bionic 18.04

[uklotzde]

• Update comments
• Update clang format settings
• Remove obsolete workaround
• Remove obsolete references in CI build
• Upgrade actions-setup-cmake in CI build

TODO:

• Update PPA build

[daschuer]

LGTM, Thank you.
What needs to be done at the PPA?

[uklotzde]

LGTM, Thank you.
What needs to be done at the PPA?

I deliberately excluded all PPA tasks, because I am not aware of all the subtleties and potential pitfalls.

[daschuer]

That's no Problem. I will take care.
I just like to know what is to do for the PPA build.
I don't see a anything.

[Holzhaus]

Maybe delete the Ubuntu 18.04 package, so that users get a warning from their package manager that the package is not updated any longer.

[uklotzde]

@daschuer Regarding the PPA: I was unsure if you want to delete all the conditionals that were required for supporting different releases with differing dependencies.

Do we distribute main builds through the PPA?? Then we definitely need to delete those 18.04 packages.

[daschuer]

Done, see: https://launchpad.net/~mixxx/+archive/ubuntu/nightlies

[daschuer]

Thank you for taking care.