Remove rhel6 from all tests

[DavidS]

RHEL6 is out of public support and we want to stop testing against it.

[codecov-io]

Codecov Report

Merging #352 (ff656d6) into main (d614fe3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #352   +/-   ##
=======================================
  Coverage   61.45%   61.45%           
=======================================
  Files           7        7           
  Lines         838      838           
=======================================
  Hits          515      515           
  Misses        323      323           

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