update java version from 1.8 to 15

[marc0olo]

closes #175

[mitch-lbw]

Reviewed and approved

[codecov-commenter]

Codecov Report

Merging #177 (1df5c67) into 3.x (746453d) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##                3.x     #177   +/-   ##
=========================================
  Coverage     13.20%   13.20%           
  Complexity      150      150           
=========================================
  Files           145      145           
  Lines          3210     3210           
  Branches         79       79           
=========================================
  Hits            424      424           
  Misses         2760     2760           
  Partials         26       26           

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