Remove "not_used" and add "active" metric statuses.

What does this MR do?

Related to #335894 (closed).

This removes an unused not_used metric status and adds active status that will be used instead of data_available and implemented.

The changes stop short at removing data_available and implemented to make sure that no new MRs come in with metric definitions containing these old statuses. Docs now encourage using active.

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

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