Allow filtering for admins in the list accounts admin API

[afechler]

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Pull request includes a sign off
• Code style is correct
  (run the linters)

[clokep]

@afechler Please sign-off on your changes.

[DMRobertson]

Aside: I wonder if this was missed in #15515?

[DMRobertson]

I'd like to see the docs and docstring phrasing changed; everything else is a nice-to-have.

[DMRobertson]

Suggested change

	Query-Parameter added to Admin-API /users to filter admins from user list.
	Add an `admins` query parameter to the [List Accounts](https://matrix-org.github.io/synapse/v1.91/admin_api/user_admin_api.html#list-accounts) [admin API](https://matrix-org.github.io/synapse/v1.91/usage/administration/admin_api/index.html), to include only admins or to exclude admins in user queries.

[DMRobertson]

Sorry, I missed the changelog earlier.

[afechler]

I have adopted the text.

[DMRobertson]

Thanks, I'm happy if CI is!

[DMRobertson]

CI passed except for a flake on 48feabf, going to merge manually.