add CAS toggle to PULL_REQUEST_TEMPLATE

Author: strtgbb

.github/PULL_REQUEST_TEMPLATE.md

@@ -39,6 +39,7 @@
 - [ ] <!---ci_regression_aggregate_functions--> Aggregate Functions (2h)
 - [ ] <!---ci_regression_alter--> Alter (1.5h)
 - [ ] <!---ci_regression_benchmark--> Benchmark (30m)
+- [ ] <!---ci_regression_cas--> CAS (content-addressed storage; Antalya only)
 - [ ] <!---ci_regression_clickhouse_keeper--> ClickHouse Keeper (1h)
 - [x] <!---ci_regression_iceberg--> Iceberg (2h)
 - [ ] <!---ci_regression_ldap--> LDAP (1h)