doc/cephfs: clarify deprecated commands and remove obsolete upgrade path

Several CephFS pages described deprecated commands and features without
clear status or replacements, and one page documented an upgrade path that
predates every supported release.

* cephfs-mirroring: correct the peer_add deprecation note to reference the
  actual replacement command, peer_bootstrap create, and link the Bootstrap
  Peers section
* kernel-features, experimental-features: state that inline data has been
  deprecated since the Octopus release and that enabling it triggers a
  health warning
* upgrading: remove the "Upgrading pre-Firefly file systems past Jewel"
  section; the tmap_upgrade command it referenced was removed in Kraken

Fixes: https://tracker.ceph.com/issues/77188
Signed-off-by: Emmanuel Ameh <eameh@contractor.linuxfoundation.org>