Remove legacy rolebinding from `build_rbac_resources`

[nightkr]

#909 changed build_rbac_resources to use the stacklet name rather than product name, but we still kept the old format in the binding to prevent outages during upgrades.

It should be removed after a grace period. (The comment says v24.3+1, but looking at the dates v25.3+1 would make more sense..)