vacuumdb: Correct comment about --force-index-cleanup.

vacuumdb: Correct comment about --force-index-cleanup.

Commit 3499df0d added a comment that incorrectly suggested that
--force-index-cleanup did not appear in the same major version as the
similar --no-index-cleanup option.  In fact, both options are new to
PostgreSQL 14.

Backpatch: 14-, where both options were introduced.