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replacement for the OpenTracing filter since OpenTracing itself is no
longer maintained nor supported:

  haproxy/haproxy#2782 (comment)

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.

Author: wtarreau

addons/ot/README

@@ -47,6 +47,12 @@ via the OpenTracing API with OpenTracing compatible servers (tracers).
 Currently, tracers that support this API include Datadog, Jaeger, LightStep
 and Zipkin.
 
+Note: The OpenTracing filter shouldn't be used for new designs as OpenTracing
+      itself is no longer maintained nor supported by its authors. A
+      replacement filter base on OpenTelemetry is currently under development
+      and is expected to be ready around HAProxy 3.2. As such OpenTracing will
+      be deprecated in 3.3 and removed in 3.5.
+
 The OT filter was primarily tested with the Jaeger tracer, while configurations
 for both Datadog and Zipkin tracers were also set in the test directory.
 

doc/configuration.txt

@@ -28490,6 +28490,13 @@ filter opentracing [id <id>] config <file>
 More detailed documentation related to the operation, configuration and use
 of the filter can be found in the addons/ot directory.
 
+Note: The OpenTracing filter shouldn't be used for new designs as OpenTracing
+      itself is no longer maintained nor supported by its authors. A
+      replacement filter base on OpenTelemetry is currently under development
+      and is expected to be ready around HAProxy 3.2. As such OpenTracing will
+      be deprecated in 3.3 and removed in 3.5.
+
+
 9.7. Bandwidth limitation
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