Remove doc references to stale gitter channel

[dstrain115]

Fixes: #6498

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.78%. Comparing base (bc3f9ff) to head (51d89db).
Report is 1 commits behind head on main.

❗ Current head 51d89db differs from pull request most recent head f49f56f. Consider uploading reports for the commit f49f56f to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6540      +/-   ##
==========================================
- Coverage   97.79%   97.78%   -0.01%     
==========================================
  Files        1124     1107      -17     
  Lines       95468    95182     -286     
==========================================
- Hits        93359    93073     -286     
  Misses       2109     2109              

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[pavoljuhas]

Thank you!