Put SAML callback URI under /_synapse/client. (#9289)

UPGRADE.rst

changelog.d/9289.removal

docs/sample_config.yaml

docs/workers.md

synapse/config/saml2_config.py

synapse/handlers/saml_handler.py

synapse/rest/synapse/client/__init__.py

synapse/rest/synapse/client/saml2/__init__.py

synapse/rest/synapse/client/saml2/metadata_resource.py

synapse/rest/synapse/client/saml2/response_resource.py