Drop Java 8 support

[msridhar]

This PR drops support for running NullAway on a JDK 8 JVM. After it lands, running NullAway will require JDK 11 or above (like current versions of Error Prone). There are various cleanups that this change enables, but to keep the PR small, I tried to do close to the minimum. In particular:

• I updated the minimum supported Error Prone version to 2.14.0, which is ~2 years old. Depending on what we decide about policy (see #882) we may want to bump this to an even more recent version.
• Update our JDK 8 test tasks to instead just test building with the most recent supported Error Prone version but running on the oldest supported version. I tested locally that this does still detect binary compatibility issues.
• Remove references to Java 11 in the jarinfer build files, as we now use that version everywhere.

We will have to change the required CI job names before landing this.

Will do other enabled cleanups in separate PRs.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 86.11%. Comparing base (5d3e264) to head (21b8999).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #961 +/- ## ============================================ - Coverage 86.12% 86.11% -0.01% Complexity 2040 2040 ============================================ Files 81 81 Lines 6708 6707 -1 Branches 1293 1293 ============================================ - Hits 5777 5776 -1 Misses 518 518 Partials 413 413 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.