Update errorprone gradle plugin to v1.2.1

[epeee]

see https://github.com/tbroyer/gradle-errorprone-plugin/releases for details

[codecov-commenter]

Codecov Report

Merging #1958 into release/3.x will not change coverage.
The diff coverage is n/a.

@@              Coverage Diff               @@
##             release/3.x    #1958   +/-   ##
==============================================
  Coverage          85.77%   85.77%           
  Complexity          2543     2543           
==============================================
  Files                318      318           
  Lines               7215     7215           
  Branches             861      861           
==============================================
  Hits                6189     6189           
  Misses               810      810           
  Partials             216      216           

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[TimvdLippe]

Thanks!