github: disable Alpine Linux CI on this release branch

Alpine is a rolling release, so its CI catches compile issues early
(e.g., library compat changes). That is useful on the development
branch, where Alpine remains enabled, but is not needed on stable
release branches. Keep workflow_dispatch so it can still be run
manually.

Fixes: CORE-16616

Author: travisdowns

.github/workflows/alpinelinux.yaml

@@ -1,7 +1,8 @@
 name: Alpine Linux
 
 on:
-  pull_request:
+  # pull_request trigger disabled on stable release branches, see CORE-16616
+  # pull_request:
   workflow_dispatch:  # Allows manual triggering
 
 concurrency: