RFR: 8345212: Since checker should better handle non numeric values

[Nizar Benalla]

Adds a new `-ignoreSince` flag to the SinceChecker, mainly to help long running projects that are not sure about `@since` version of an API.
The value of `@since` in `@since <ignoredString>` will be the release version, as I assume that this is a new API in a different OpenJDK project. This flag is not meant to be used in the JDK.

I also improved error message when the actual `@since` value doesn’t match expectations. It used to say `@since version 9; should be <number>` but it now explains the error more clearly.

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Commit messages:
 - better handle formats like 6u25, rewrite function to be simpler
 - respond to feedback by chen
 - add -ignoreSince flag to sinceChecker

Changes: https://git.openjdk.org/jdk/pull/23447/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23447&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345212
  Stats: 66 lines in 1 file changed: 45 ins; 14 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/23447.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23447/head:pull/23447

PR: https://git.openjdk.org/jdk/pull/23447