CompatHelper: bump compat for Symbolics to 6 for package test, (keep existing compat)

[github-actions]

This pull request changes the compat entry for the Symbolics package from 1 - 5 to 1 - 5, 6 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.

[schillic]

x-ref #3635
The new version of Symbolics breaks the nice parsing of expressions that we use it for:
Hyperplane(2x + 3y == 5) now becomes Hyperplane([3.0, 2.0], 5.0).
I had asked for a way to get the sorted variables a while ago, but so far there is no solution. Until then, this PR is on hold.