[lworld] RFR: 8344541: [lworld] jdk/tools/sincechecker/modules/java_base/CheckSince_javaBase.java finds "@since" version errors

[David Simms]

Added `--ignoreSince <string>` option to SinceChecker to allow the project keep
using the token "Valhalla" as `since` version until integration time, at which
point, a simple replace `@since Valhalla` with the appropriate version.

This also requires new API to use the since tag, since the checker still
requires it (even if its just a token).

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Commit messages:
 - Merge branch 'lworld' into 8344541
 - 8344541: [lworld] jdk/tools/sincechecker/modules/java_base/CheckSince_javaBase.java finds "@since" version errors

Changes: https://git.openjdk.org/valhalla/pull/1312/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1312&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344541
  Stats: 30 lines in 6 files changed: 26 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/valhalla/pull/1312.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1312/head:pull/1312

PR: https://git.openjdk.org/valhalla/pull/1312