Run Java 14 on our CI

[smarter]

Now that #8838 is fixed, we should be able to run Java 14. Since it tends to catch more things than Java 8 and 11 I propose dropping Java 11, making Java 14 the default, and keeping Java 8 only for merges/nightlies/releases (in particular published jars should always be produced using Java 8 to make sure no Java 9+ only dependency slips in).

[liufengyun]

This is partly done for #9181, but only on Java 11. We need to update the docker image to switch to Java 14.

• Due to Java 9+: staging classpath handling #9500, we still need to run bootstrapped tests on Java 8
• Due to API compat (27f94f8), we need to run community projects on Java 8

[smarter]

We replaced Java 11 by 14 in 9dc145d