Synchronize default Java version on latest runners

[sormuras]

Three different defaults is ... odd.

• 11.0.24+8 (default) on Ubuntu
• 21.0.4+7.0 (default) on macOS
• 8.0.422+5 (default) on Windows

What about setting all to "21"?

[sormuras]

Related to:

• https://github.com/actions/runner-images/discussions/10059
• https://github.com/actions/runner-images/discussions/9266

[erik-bershel]

Yes, it does look strange, but in fact the main reason is that Java users react extremely negatively to default version updates, which forces us to take such a step most often only when a new OS version is released.