CompatHelper: bump compat for Symbolics in [extras] to 7, (keep existing compat)

[github-actions]

This pull request changes the compat entry for the Symbolics package from 5 to 5, 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 98.27%. Comparing base (461c9e2) to head (7851582).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #34   +/-   ##
=======================================
  Coverage   98.27%   98.27%           
=======================================
  Files           1        1           
  Lines         232      232           
=======================================
  Hits          228      228           
  Misses          4        4           

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