doc: clean up PendingReleaseNotes for reef

This PR removes all of the release notes up to
version 17 (Quincy) inclusive.

As far as I know, for as long as I have worked
on the Ceph project, we have never cleaned up
the release notes from release to release.

Until now.

Signed-off-by: Zac Dover <[email protected]>

Author: Zac Dover