"Requested operation has changed during the UI authentication session." shouldn't be raised if only the query string changes

[matrixbot]

This issue has been migrated from #7722.

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matrix-org/synapse#7483 changed the condition under which the "Requested operation has changed during the UI authentication session." error raised so that it's only raised if the (method, uri) changes during UIA.

This means that if e.g. I initiate a registration with POST /register but ends it with POST /register?kind=user, which is the same operation, the registration fails.

Ideally this condition should be changed to omit the query string from the uri, but we should first check if that doesn't introduce any issue (e.g. if the UIA operation is passed in the query string in some cases - which I don't think it is, but we should probably make sure of that).