Fix Helm chart executor label to support executor aliases (#67762)

* Fix Helm chart executor label to support executor aliases

The scheduler Deployment/StatefulSet renders the executor option into a
Kubernetes label and only sanitized the comma separator used between
multiple executors. Executor aliases (name:ExecutorClass) contain a
colon, which is not a valid label character, so a value such as
CeleryExecutor,harvest_exec:KubernetesExecutor produced an invalid label
and the scheduler failed to deploy.

Replace the colon with a hyphen alongside the existing comma handling so
aliased and multi-executor configurations render a valid label.

* Add newsfragment for executor alias label fix

* Remove newsfragment; not needed for a bugfix per AGENTS.md