Suggested compat bounds for stdlibs at odds with PSA recommendation

From https://discourse.julialang.org/t/psa-compat-requirements-in-the-general-registry-are-changing/104958
> For now you should simply **match** the version of your Julia compat entry, for all your standard library compat entries.

Last night, TagBot has started suggesting compat bounds for several stdlibs in several respositories, e.g., [PackageCompiler.jl](https://github.com/JuliaLang/PackageCompiler.jl/pull/882/files) or [ReadVTK.jl](https://github.com/JuliaVTK/ReadVTK.jl/pull/46/files). There, however, it seems like the suggested compat bound is always just `"1"`, even though the Julia compat is `"1.6"`.

This mismatch between TagBot and the PSA recommendation is at least weird for package maintainers who are not in the loop regarding the further development of upgradeable stdlibs: Should we just wait for TagBot to update everything to `"1"`? Or should we go ahead and manually set it to the Julia compat?

And is this mismatch just due to a limitation of TagBot not being able to extract the Julia compat for use in stdlibs, or is it an actual bug?

Either way, it would be great to see some guidance on this in the TagBot README and/or the PSA.

cc @vchuravy

Start with the TagBot README and compare its stated compat-bound behavior with the Julia compat guidance quoted in this issue. Review the linked PackageCompiler.jl and ReadVTK.jl pull requests to establish the observed mismatch; done means documenting the correct recommendation and clarifying whether the behavior is intentional or a bug.

Written by the indexing model from the issue text.