Remove deprecated annotations

[rainest]

What this PR does / why we need it:
Removes deprecated annotations.

Special notes for your reviewer:
Tests still use literal "konghq.com/regex-priority": 3 style test values instead of AnnotationPrefix + RegexPriorityKey: 3 package constants. We're kinda moving towards the latter in general, though you could make the argument that we want the literal strings so tests complain in the event that we change something: while that works in code using the constants, users won't have changed their annotations in advance if we release such a change, so those literals could serve as a useful canary to make sure that we call out breaking changes if we do change these in the future.

Several tests outside the annotations package referenced annotations.DeprecatedPluginsKey and now instead reference annotations.AnnotationPrefix + annotations.PluginsKey. For consistency, and lack of an obvious reason not to, the annotations package now exports both the modern prefix and all keys, even though not all keys are currently used in non-annotation tests.

[hbagdi]

Looks good, deferring approval to @mflendrich.

[mflendrich]

LGTM but please fix the dead link (see my other comment).

that we want the literal strings so tests complain in the event that we change something

Testing our application's contract (in particular, that user-facing annotations work as expected by users) sounds great. I think that a good place for contract testing would be on a higher level than here, though - otherwise we're likely to implement a change-detector test with all the problems as mentioned in this ToTT blog post.

I think that the end-to-end test (see #869) could be a good start for such contract tests.