Internal Changes

----------------

- Update federation routing logic to check .well-known before SRV ([\#4539](https://github.com/matrix-org/synapse/issues/4539))
- Improve performance of handling servers with invalid .well-known ([\#4542](https://github.com/matrix-org/synapse/issues/4542))
- Treat an invalid .well-known file the same as an absent one ([\#4544](https://github.com/matrix-org/synapse/issues/4544))
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