CompatHelper: bump compat for Symbolics in [weakdeps] to 6, (keep existing compat)

[github-actions[bot]]

This pull request changes the compat entry for the Symbolics package from 5.14 to 5.14, 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.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.67%. Comparing base (df099d1) to head (9a0e9fb).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #24 +/- ## ======================================= Coverage 89.67% 89.67% ======================================= Files 16 16 Lines 1394 1394 ======================================= Hits 1250 1250 Misses 144 144 ```

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