RFR: 8332102: Add `@since` to package-info of `jdk.security.jarsigner`

[Nizar Benalla]

Code cleanup. The package was added back in [8056174](https://bugs.openjdk.org/browse/JDK-8056174).
Thanks to anyone reviewing this change.

-------------

Commit messages:
 - add an `@since` and modify Copyright header

Changes: https://git.openjdk.org/jdk/pull/19191/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19191&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332102
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/19191.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19191/head:pull/19191

PR: https://git.openjdk.org/jdk/pull/19191