CompatHelper: add new compat entry for MCMCChains at version 7 for package test, (keep existing compat)

[github-actions]

This pull request sets the compat entry for the MCMCChains package to 7 for package test.
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.
Note: Consider registering a new release of your package immediately after merging this PR, as downstream packages may depend on this for tests to pass.

[github-actions]

Turing.jl documentation for PR #2824 is available at:
https://TuringLang.github.io/Turing.jl/previews/PR2824/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.07%. Comparing base (18eedab) to head (240aa20).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2824      +/-   ##
==========================================
+ Coverage   84.61%   85.07%   +0.46%     
==========================================
  Files          23       23              
  Lines        1514     1514              
==========================================
+ Hits         1281     1288       +7     
+ Misses        233      226       -7     

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[shravanngoswamii]

@yebai I am happy to merge this, but how long are we planning to keep MCMCChains around in the ecosystem? Many people might still be using it, but maybe we should start phasing it out. And we should start improving guidance for users toward FlexiChains -- which I suppose is already ongoing.

[yebai]

@shravanngoswamii, happy for you to figure out a sensible plan for it -- FlexiChains is still new and likely requires time to mature/improve.

[shravanngoswamii]

Self reviewed my changes in this PR

[shravanngoswamii]

happy for you to figure out a sensible plan for it

Looking at FlexiChains -- it still needs some work on visualization's support, and once FlexiChains has reasonable parity and we can add a soft deprecation notice, like maybe print a one-time @info when chain_type=MCMCChains.Chains is used.

[shravanngoswamii]

@yebai Is it fine if I merge these kinds of PRs without 2nd reviews?

[yebai]

Thanks @shravanngoswamii!

[sunxd3]

happy to review these PRs if you need a second opinion

[shravanngoswamii]

Thanks @sunxd3, will ping you whenever I update them!

Feel free to merge this one!