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

[github-actions]

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

[github-actions]

Benchmark Results

	main	1466b09...	main/1466b094b8e7e8...
assemble/basic bilinear/mass matrix	15.5 ± 0.12 ms	15.5 ± 0.084 ms	0.997
assemble/basic bilinear/stiffness matrix	0.0826 ± 0.003 s	0.0799 ± 0.00017 s	1.03
assemble/poisson DG/AffineFESystem	0.0531 ± 0.0065 s	0.0536 ± 0.0067 s	0.991
assemble/poisson DG/a_Γ(u, v)	0.0383 ± 0.0016 s	0.0383 ± 0.0049 s	1
assemble/poisson DG/a_Γb(u, v)	7.39 ± 0.15 ms	7.37 ± 0.097 ms	1
assemble/poisson DG/a_Ω(u, v)	7.65 ± 0.019 ms	7.65 ± 0.019 ms	1
assemble/poisson DG/l_Γb(v)	0.114 ± 0.039 ms	0.122 ± 0.039 ms	0.928
assemble/poisson DG/l_Ω(v)	12.7 ± 0.18 μs	12.8 ± 0.15 μs	0.992
covo/Quad/explicit_step	0.0809 ± 0.0014 s	0.0785 ± 0.00075 s	1.03
covo/Quad/integral_surface	0.0509 ± 0.00015 s	0.0516 ± 0.00013 s	0.986
covo/Quad/integral_volume	20.5 ± 0.04 ms	20 ± 0.063 ms	1.03
covo/TriQuad/explicit_step	0.393 ± 0.034 s	0.391 ± 0.033 s	1.01
covo/TriQuad/integral_surface	0.303 ± 0.0036 s	0.306 ± 0.0032 s	0.989
covo/TriQuad/integral_volume	0.0746 ± 0.0055 s	0.0744 ± 0.006 s	1
entities/edges2nodes	1.56 ± 0.01 ns	1.56 ± 0.01 ns	1
entities/nedges	1.55 ± 0.01 ns	1.56 ± 0.01 ns	0.995
entities/nnodes	3.41 ± 0.01 ns	3.41 ± 0.01 ns	1
entities/nodes	1.55 ± 0.01 ns	1.56 ± 0.01 ns	0.994
mesh/todo	1.55 ± 0.01 ns	1.55 ± 0.01 ns	1
time_to_load	2.71 ± 0.0022 s	2.71 ± 0.0045 s	0.999

Benchmark Plots

A plot of the benchmark results have been uploaded as an artifact to the workflow run for this PR.
Go to "Actions"->"Benchmark a pull request"->[the most recent run]->"Artifacts" (at the bottom).