Stabilize support for MSC3952: Intentional mentions. (#15520)

changelog.d/15520.feature

rust/benches/evaluator.rs

rust/src/push/base_rules.rs

rust/src/push/evaluator.rs

rust/src/push/mod.rs

stubs/synapse/synapse_rust/push.pyi

synapse/api/constants.py

synapse/config/experimental.py

synapse/events/validator.py

synapse/push/bulk_push_rule_evaluator.py

synapse/rest/client/versions.py

synapse/storage/databases/main/push_rule.py

tests/push/test_bulk_push_rule_evaluator.py