Docs: Confirm if Observability plugin was actually deprecated, and if it was update the docs

[abrennan89]

Expected Behavior

https://knative.dev/docs/install/any-kubernetes-cluster/ contains only up to date information about components that can be installed.

Actual Behavior

It mentions an Observability plugin that was deprecated in 0.14. The oldest currently published version of the docs is now 0.14, so this could be removed from all the documentation if it actually was deprecated at 0.14.

Steps to Reproduce the Problem

1. See https://knative.dev/docs/install/any-kubernetes-cluster/
2. See https://knative.dev/docs/install/any-kubernetes-cluster/#installing-the-observability-plugin

[abrennan89]

Updated older branches

2905 (0.15)

2907 (0.16)

2908 (0.17)

2909 (0.18)

[evankanderson]

Other refs to the monitoring bundle:

https://knative.dev/docs/serving/fluentd-requirements/
https://knative.dev/docs/serving/accessing-metrics/
https://knative.dev/docs/serving/accessing-traces/
https://knative.dev/docs/serving/samples/telemetry-go/index.html
https://knative.dev/docs/eventing/accessing-traces/

[evankanderson]

I have some work on the fluentd item, but nothing for traces (which may just need "if you have Jaeger/Zipkin" rather than "here's a Jaeger/Zipkin").

[dprotaso]

It's now been dropped in serving - so it'll be part of the 0.19 release

https://github.com/knative/serving/pull/9807