airflow docs: update some outdated recommendations

[mobuchowski]

What does this PR do?

Docs only - it moves to recommend to use metrics section in configuration rather than scheduler, as this was moved to metrics a few years back and Airflow 1 usage is now disappearing.

Also, removes one metric that never worked and was removed in newer releases: https://github.com/apache/airflow/blob/70a8a8a9c3696e7a63aad5a0c11bbc4d8578faa9/RELEASE_NOTES.rst#the-gauge-schedulertasksrunning-no-longer-exist-30374

Motivation

Noticing outdated recommendations while reading docs.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[janine-c]