Update the SSO username picker template to comply with SIWA guidelines (#12210)

changelog.d/12210.misc

docs/sample_config.yaml

docs/templates.md

synapse/config/oidc.py

synapse/handlers/oidc.py

synapse/handlers/sso.py

synapse/res/templates/sso_auth_account_details.html

synapse/rest/synapse/client/pick_username.py