Merge tag 'v1.40.0rc2' into develop

Synapse 1.40.0rc2 (2021-08-04)
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Bugfixes
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- Fix the `PeriodicallyFlushingMemoryHandler` inhibiting application shutdown because of its background thread. ([\#10517](https://github.com/matrix-org/synapse/issues/10517))
- Fix a bug introduced in Synapse v1.40.0rc1 that could cause Synapse to respond with an error when clients would update read receipts. ([\#10531](https://github.com/matrix-org/synapse/issues/10531))

Internal Changes
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- Fix release script to open the correct URL for the release. ([\#10516](https://github.com/matrix-org/synapse/issues/10516))