Add script for bulk-clearing room retention policies in the admin FAQ

This issue has been migrated from [#13528](https://github.com/matrix-org/synapse/issues/13528).

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**Description:**

@babolivier has a nice script for pruning retention policies [here](https://gist.github.com/babolivier/64b6b539ce612f44db3171a4c3d4ecaf) that can be useful in other contexts, too (e.g. when bulk-changing room history visibility settings). It might be good to include this in the ["Admin FAQ" section](https://matrix-org.github.io/synapse/latest/usage/administration/admin_faq.html) of the docs.

Putting this here as an issue for discussion first in case somebody else knows a better place to host this.