(Re)set minimal Java version to (again) be 8 instead of 11 (fixes #142)

[vorburger]

This should fix #142.

@mosesn @duttonw @thomasbao12 LGTY?

[vorburger]

Duh! While this worked locally, as-is it fails on CI, twice, for Java 8 due to: Unrecognized option: --add-exports

and on Java 21 due to: source value 8 is obsolete and will be removed in a future release

[vorburger]

for Java 8 due to: Unrecognized option: --add-exports

I figured out that this is because of https://errorprone.info/docs/installation, hear hear:

Please note that Error Prone must be run on JDK 11 or newer. (It can still be used to build Java 8 code by setting the appropriate -source / -target / -bootclasspath flags.)

Ergo, the GH CI Action matrix build cannot run under Java 8, but only Java 11+ ... but we can and are still using source/target/release 8 instead 11 in the pom.xml; I've done the needful, and this seems to work.

and on Java 21 due to: source value 8 is obsolete and will be removed in a future release

I figured out that this can be fixed with javac option -Xlint:all,-options; I've done the needful, and this seems to work.