CompatHelper: bump compat for MCMCChains in [weakdeps] to 7, (keep existing compat)

[github-actions]

This pull request changes the compat entry for the MCMCChains package from 6 to 6, 7.
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.
✅ Project coverage is 87.47%. Comparing base (bfd665d) to head (1b82f78).
⚠️ Report is 82 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #347   +/-   ##
=======================================
  Coverage   87.47%   87.47%           
=======================================
  Files         107      107           
  Lines        2658     2658           
=======================================
  Hits         2325     2325           
  Misses        333      333           

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