Change the default java `source_version` / `target_version` to `11`

Using a language version of 8 and running with jdk21 (or later) results in:

```
warning: [options] source value 8 is obsolete and will be removed in a future release
warning: [options] target value 8 is obsolete and will be removed in a future release
warning: [options] To suppress warnings about obsolete options, use -Xlint:-options.
```
PiperOrigin-RevId: 683542164
Change-Id: Icc9600af2543720ff98a9d695cdd526f59612b9c