Docs: fix notes on the MR approvals API doc

Problem to solve

All "introduced in" notes in the MR approvals API doc should be fixed since they're against the guidelines.

Further details

Notes like:

>**Note:** This API endpoint is only available on 12.3 Starter and above.

should be changed to:

> [Introduced](link-to-issue) in [GitLab Starter](https://about.gitlab.com/pricing/) 12.3.

Or, when not clear what was introduced, change to:

> _Update project approval rules_ API was [introduced](link-to-issue) in [GitLab Starter](https://about.gitlab.com/pricing/) 12.3.

Or, if the issue is unknown, at least:

> Introduced in [GitLab Starter](https://about.gitlab.com/pricing/) 12.3.

Who can address the issue

Anyone. It's ~"Good for 1st time contributors" and Community contributions.

Other links/references

I've identified this issue when reviewing !22754 (merged).