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

[github-actions]

This pull request changes the compat entry for the AbstractMCMC package from 3, 4 to 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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (cf5cfb1) 2.17% compared to head (57ec0f7) 7.85%.

Additional details and impacted files

@@           Coverage Diff            @@
##            main    #321      +/-   ##
========================================
+ Coverage   2.17%   7.85%   +5.68%     
========================================
  Files         20      20              
  Lines       1790    1794       +4     
========================================
+ Hits          39     141     +102     
+ Misses      1751    1653      -98     

see 1 file with indirect coverage changes

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