Fixes issue

Describe the change

Modernize Go to the 1.22 specs:

• Use "any" instead of an empty interface.
• Use range over an integer for for loops.
• Remove shadow variables in loops now that Go no longer reuses the variable.
• Use "slices.Contains", "slices.Delete", "slices.Equal", "slices.Index", "slices.SortFunc".
• Use "cmp.Or", "min", and "max".
• Use "fmt.Appendf" instead of "Sprintf" for generating a byte slice.
• Use "errors.Join" or "fmt.Errorf" with multiple "%w" for multiple errors.

Additionally, use modern regclient features:

• Use "ref.SetTag", "ref.SetDigest", and "ref.AddDigest".
• Call "regclient.ManifestGet" using "WithManifestDesc" instead of setting the digest on the reference.

How to verify it

There are very few user visible changes (at most some error messages may be improved).

make test

Given the scope of the changes, there is potential for introducing bugs, but it's also likely quite a few other bugs have been resolved by using language built-in features.

Changelog text

• Chore: Modernize Go to the 1.22 specs.
• Feat: Add ref.AddDigest method that does not unset the tag.

Please verify and check that the pull request fulfills the following requirements

• Tests have been added or not applicable
• Documentation has been added, updated, or not applicable
• Changes have been rebased to main
• Multiple commits to the same code have been squashed