CompatHelper: bump compat for P4est_jll to 2, (keep existing compat)

[github-actions]

This pull request changes the compat entry for the P4est_jll package from =2.8.1 to =2.8.1, 2.
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.

[JoshuaLampert]

JuliaPackaging/Yggdrasil#9878 set the p4est version to v2.8.6 and enabled OpenMPI. The latter had the consequence that the current version of P4est_jll.jl is broken on Windows. JuliaPackaging/Yggdrasil#9911 should have fixed that by disabling OpenMPI again. However, CI failed, which (I guess) is the reason why no new P4est_jll.jl version has been registered after that PR. I don't know why CI failed. It was successful (after a restart) in the PR. Is there a way to rerun the Yggdrasil pipeline for the PR JuliaPackaging/Yggdrasil#9911, which hopefully succeeds, such that P4est_jll.jl gets updated? Once we managed that and CI is successful again here, we can consider setting the compat bound as P4est_jll = "=2.8.6". This should also close #76. I am not sure though whether the issues discussed in #76 are addressed, such that we are ready to update the p4est version.

[sloede]

Is there a way to rerun the Yggdrasil pipeline for the PR JuliaPackaging/Yggdrasil#9911, which hopefully succeeds, such that P4est_jll.jl gets updated?

Best to ping Mosè Giordano about this either in the Yggdrasil PR or the #binarybuilder channel in the Julia Slack.

[ranocha]

Please also note #76 and the discussions linked there

[JoshuaLampert]

Tests are still failing on Windows with P4est_jll.jl@2.8.7 😬