Drop support for Java 8

[FlorianHockmann]

Java 8 is already EOL since April 2022, but at least still receives security updates until 2030. Source Since our general policy is to not support versions that are already EOL, we should also follow this for the Java versions. But this is of course a breaking change and we should not do this earlier than the first major release after 1.0.0 as we only add official support for Java 11 in that release.

Follow-ups after dropping support for Java 8:

• Update caffeine to v3. (This requires Java 11.)
• Update Mockito to v5. (also requires Java 11.)
• Update checkstyle to v10.
• Update audience-annotations to 0.15.0 (also requires Java 11)