Merge branch 'develop' into cross-signing_federation

.buildkite/postgres-config.yaml

.buildkite/scripts/create_postgres_db.py

.buildkite/scripts/test_synapse_port_db.sh

.buildkite/sqlite-config.yaml

.buildkite/test_db.db

AUTHORS.rst

CHANGES.md

CONTRIBUTING.rst

UPGRADE.rst

changelog.d/6140.misc

changelog.d/6218.misc

changelog.d/6240.misc

changelog.d/6247.bugfix

changelog.d/6248.misc

changelog.d/6250.misc

changelog.d/6251.misc

changelog.d/6253.bugfix

changelog.d/6257.doc

changelog.d/6263.misc

changelog.d/6269.misc