ci: drop unnecessary push triggers from SundialKit(Stream)

Mirror the AtLeast CI tidy (ae46621): the push trigger now only fires on
main. Removes the atleast-beta.6 push branch and the version-tag push
trigger from both SundialKit and SundialKitStream workflows; pull_request
triggers are unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: leogdion

.github/workflows/SundialKit.yml

@@ -3,9 +3,6 @@ on:
   push:
     branches:
       - main
-      - atleast-beta.6
-    tags:
-      - 'v[0-9]*.[0-9]*.[0-9]*'
     paths-ignore:
       - '**.md'
       - 'Docs/**'