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

org.eclipse.jgit/src/org/eclipse/jgit/lib/PersonIdent.java

@@ -408,7 +408,7 @@ public TimeZone getTimeZone() {
 	 * Get the time zone id
 	 *
 	 * @return the time zone id
-	 * @since 7.1
+	 * @since 6.1
 	 */
 	public ZoneId getZoneId() {
 		return tzOffset;
@@ -496,4 +496,3 @@ public String toString() {
 		return r.toString();
 	}
 }
-