doc/rados/operations: Strengthen cache-tiering deprecation notice

The existing warning hedged with "this does not mean it will certainly
be removed", softening the message for a deprecated feature. Operators
scanning the page may begin a how-to setup without reading past the
introductory prose.

- Remove the hedging language; state plainly that cache tiering may be
  removed without further notice
- Add a bold "Do not deploy new cache tiers" directive
- Add a note immediately before the how-to content scoping the page
  to existing deployments only, with a pointer to preferred alternatives

Fixes: https://tracker.ceph.com/issues/77184

Signed-off-by: Emmanuel Ameh <eameh@contractor.linuxfoundation.org>