CompatHelper: bump compat for AbstractMCMC to 5, (keep existing compat)

[github-actions]

This pull request changes the compat entry for the AbstractMCMC package from 2, 3, 4 to 2, 3, 4, 5.
This keeps the compat entries for earlier versions.

Note: I have not tested your package with this new compat entry.
It is your responsibility to make sure that your package tests pass before you merge this pull request.

[github-actions]

Pull Request Test Coverage Report for Build 6647726790

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 95.864%

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Totals
Change from base Build 6618374054:	0.0%
Covered Lines:	394
Relevant Lines:	411

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💛 - Coveralls

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (67d3496) 95.86% compared to head (21e3675) 95.86%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #88   +/-   ##
=======================================
  Coverage   95.86%   95.86%           
=======================================
  Files           8        8           
  Lines         411      411           
=======================================
  Hits          394      394           
  Misses         17       17           

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