Replace returnValue with return (#5736)

changelog.d/5736.misc

docs/log_contexts.rst

synapse/api/auth.py

synapse/api/filtering.py

synapse/app/frontend_proxy.py

synapse/app/homeserver.py

synapse/appservice/__init__.py

synapse/appservice/api.py

synapse/appservice/scheduler.py

synapse/crypto/keyring.py

synapse/events/builder.py

synapse/events/snapshot.py

synapse/events/third_party_rules.py

synapse/events/utils.py

synapse/federation/federation_base.py

synapse/federation/federation_client.py

synapse/federation/federation_server.py

synapse/federation/sender/per_destination_queue.py

synapse/federation/sender/transaction_manager.py

synapse/federation/transport/client.py