CompatHelper: bump compat for "MCMCChains" to "7.6"

[github-actions[bot]]

This pull request changes the compat entry for the MCMCChains package from 5.6, 6.0 to 5.6, 6.0, 7.6.

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.