Add Ruby 3.1 to CI, build gem during CI

[bastelfreak]

No description provided.

[codecov]

Codecov Report

Merging #79 (ef65f0f) into master (93bfef3) will not change coverage.
The diff coverage is n/a.

❗ Current head ef65f0f differs from pull request most recent head ae2b76f. Consider uploading reports for the commit ae2b76f to get more accurate results

@@           Coverage Diff           @@
##           master      #79   +/-   ##
=======================================
  Coverage   27.86%   27.86%           
=======================================
  Files           2        2           
  Lines          61       61           
=======================================
  Hits           17       17           
  Misses         44       44           

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[JanKoppe]

LGTM