Fix CompatHelper so that you don't get a duplicate PR every day

[DilumAluthge]

This PR fixes the CompatHelper config in this repo to use the latest recommend config.

Thus, among other improvements, you won't get a duplicate PR every day.

Since you use GitHub Actions CI, the following line from this PR ensures that CI gets triggered on PRs made by CompatHelper:

COMPATHELPER_PRIV: ${{ secrets.DOCUMENTER_KEY }}

[DilumAluthge]

@tkf It would be great to merge this as soon as possible.

cc: @giordano

[DilumAluthge]

@tkf I've made a few other PRs to update the CompatHelper setup on some of your other repos.

If you want to update the CompatHelper setup on a whole bunch of repos at once, you might consider the MassInstallAction.jl package. E.g.:

julia> using MassInstallAction

julia> workflow = MassInstallAction.compat_helper()

julia> MassInstallAction.install(workflow,
           "tkf";
           token = ENV["MY_GITHUB_TOKEN"],
           cc = ["tkf"],
       )

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[DilumAluthge]

After merging this PR, I would go through and close all of the currently open CompatHelper PRs. Then, the next time CompatHelper runs, it will only open PRs if they need to be opened.

[DilumAluthge]

From Mergify:

GitHub App like Mergify are not allowed to merge pull request where .github/workflows is changed.

This pull request must be merged manually.