DOC: ot: mention planned deprecation of the OT filter

DOC: ot: mention planned deprecation of the OT filter

Miroslav mentioned below that he's currently working on an OpenTelemetry
replacement for the OpenTracing filter since OpenTracing itself is no
longer maintained nor supported:

  https://github.com/haproxy/haproxy/issues/2782#issuecomment-2493576327

Given that he aims for 3.2, let's already settle on an upcoming deprecation
of the filter for 3.3 with a removal for 3.5. This will leave time to finish
the development and permit users to switch smoothly. At this point no warning
is emitted (since the users have no alternative) but better mention this plan
in the doc to make them aware of future changes.