Deprecate `asdf.testing.helpers.format_tag` and `asdf.versioning.AsdfSpec`

[braingram]

Description

This PR deprecates asdf.testing.helpers.format_tag (and replaces it's one use with a string) and asdf.versioning.AsdfSpec (which is unused).

Fixes: #1773

Checklist:

• [ ] pre-commit checks ran successfully
• [ ] tests ran successfully
• [ ] for a public change, a changelog entry was added
• [ ] for a public change, documentation was updated
• [ ] for any new features, unit tests were added

[braingram]

The downstream dkist error is unrelated and looks to be caused by a gwcs version comparison in one of their tests.

[braingram]

Thanks!

The only docs I also see are the auto-generated ones: https://asdf.readthedocs.io/en/latest/search.html?q=format_tag&check_keywords=yes&area=default

I don't quite know the whole history of format_tag but looking through various git blames:

• was moved from asdf.types (when that was deprecated)
• was moved from asdf.asdftypes (when that was deprecated)
• was added in the first "skeleton" of the project

but never appears to be documented beyond the docstring and auto-generated docs. It seems to always have been a way to construct a uri string from several string (or AsdfVersion, when that was added) components.