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

[Nizar Benalla]

On Tue, 4 Feb 2025 19:11:07 GMT, Nizar Benalla <nbenalla at openjdk.org> wrote:

> 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.

Thanks for the review

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PR Comment: https://git.openjdk.org/jdk/pull/23447#issuecomment-2640285167