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 Develop a policy for how long we support Error Prone versions #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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@             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              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.