PersonIdent: Revert @since of #getZoneId

In [1], the @since tag of #getZoneId was updated to 7.1 by
mistake. The implementation of the method is different but the API
hasn't changed.

Revert the tag to 6.1, when the method was introduced.

[1] https://gerrithub.io/c/eclipse-jgit/jgit/+/1204142/9/org.eclipse.jgit/src/org/eclipse/jgit/lib/PersonIdent.java

Change-Id: If71d763ac28d4ec02bfebb1e65f56227f44e027d