Implements MSC4041 behind an experimental configuration flag.

- Add configuration setting for CAS protocol version. Contributed by Aurélien Grimpard. ([\#15816](#15816))
- Suppress notifications from message edits per [MSC3958](matrix-org/matrix-spec-proposals#3958). ([\#16113](#16113))
- Return a `Retry-After` with `M_LIMIT_EXCEEDED` error responses. ([\#16136](#16136))
- Add `last_seen_ts` to the [admin users API](https://matrix-org.github.io/synapse/latest/admin_api/user_admin_api.html). ([\#16218](#16218))
- Improve resource usage when sending data to a large number of remote hosts that are marked as "down". ([\#16223](#16223))

- Fix IPv6-related bugs on SMTP settings, adding groundwork to fix similar issues. Contributed by @evilham and @telmich (ungleich.ch). ([\#16155](#16155))
- Fix a spec compliance issue where requests to the `/publicRooms` federation API would specify `include_all_networks` as a string. ([\#16185](#16185))
- Fix inaccurate error message while attempting to ban or unban a user with the same or higher PL by spliting the conditional statements. Contributed by @leviosacz. ([\#16205](#16205))
- Fix a rare bug that broke looping calls, which could lead to e.g. linearly increasing memory usage. Introduced in v1.90.0. ([\#16210](#16210))
- Fix a long-standing bug where uploading images would fail if we could not generate thumbnails for them. ([\#16211](#16211))
- Fix a long-standing bug where we did not correctly back off from servers that had "gone" if they returned 4xx series error codes. ([\#16221](#16221))

- Update links to the [matrix.org blog](https://matrix.org/blog/). ([\#16008](#16008))
- Document which [admin APIs](https://matrix-org.github.io/synapse/latest/usage/administration/admin_api/index.html) are disabled when experimental [MSC3861](matrix-org/matrix-spec-proposals#3861) support is enabled. ([\#16168](#16168))
- Document [`exclude_rooms_from_sync`](https://matrix-org.github.io/synapse/v1.92/usage/configuration/config_documentation.html#exclude_rooms_from_sync) configuration option. ([\#16178](#16178))

- Prepare unit tests for Python 3.12. ([\#16099](#16099))
- Fix nightly CI jobs. ([\#16121](#16121), [\#16213](#16213))
- Describe which rate limiter was hit in logs. ([\#16135](#16135))
- Simplify presence code when using workers. ([\#16170](#16170))
- Track per-device information in the presence code. ([\#16171](#16171), [\#16172](#16172))
- Stop using the `event_txn_id` table. ([\#16175](#16175))
- Use `AsyncMock` instead of custom code. ([\#16179](#16179), [\#16180](#16180))
- Improve error reporting of invalid data passed to `/_matrix/key/v2/query`. ([\#16183](#16183))
- Task scheduler: add replication notify for new task to launch ASAP. ([\#16184](#16184))
- Improve type hints. ([\#16186](#16186), [\#16188](#16188), [\#16201](#16201))
- Bump black version to 23.7.0. ([\#16187](#16187))
- Log the details of background update failures. ([\#16212](#16212))
- Cache device resync requests over replication. ([\#16241](#16241))

* Bump anyhow from 1.0.72 to 1.0.75. ([\#16141](#16141))
* Bump furo from 2023.7.26 to 2023.8.19. ([\#16238](#16238))
* Bump phonenumbers from 8.13.18 to 8.13.19. ([\#16237](#16237))
* Bump psycopg2 from 2.9.6 to 2.9.7. ([\#16196](#16196))
* Bump regex from 1.9.3 to 1.9.4. ([\#16195](#16195))
* Bump ruff from 0.0.277 to 0.0.286. ([\#16198](#16198))
* Bump sentry-sdk from 1.29.2 to 1.30.0. ([\#16236](#16236))
* Bump serde from 1.0.184 to 1.0.188. ([\#16194](#16194))
* Bump serde_json from 1.0.104 to 1.0.105. ([\#16140](#16140))
* Bump types-psycopg2 from 2.9.21.10 to 2.9.21.11. ([\#16200](#16200))
* Bump types-pyyaml from 6.0.12.10 to 6.0.12.11. ([\#16199](#16199))