Text for 405 errors is a lie

This issue has been migrated from [#6894](https://github.com/matrix-org/synapse/issues/6894).

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When you send a GET request to an endpoint that only supports POST, or vice versa, synapse sends back an error that looks like:

> # Method Not Allowed
> Your browser approached me (at /_matrix/saml2/authn_response) with the method "GET". I only allow the method HEAD here.

We hardly support HEAD anywhere, and it doesn't say anything about POST.

(This actually only happens on a subset of endpoints, such as the media repo ones. For others we give back a 400, which I think is fine).

Related: https://github.com/matrix-org/synapse/issues/6746

Start by reproducing a GET request against /_matrix/saml2/authn_response and compare the 405 response with the media repository endpoints mentioned in the issue. Trace the request-method handling and verify that the error names the methods the endpoint actually allows, with behavior covered for the affected endpoints.

Written by the indexing model from the issue text.