Remove tag trigger from mark-stable workflow

Stop running the mark-stable workflow on v* tag pushes so it executes
only on branch updates. This prevents duplicate or unintended runs
after version tags are created as part of the release process.

https://chatgpt.com/share/6988bef0-1a0c-800f-93df-7a6c1bdc0331

.github/workflows/mark-stable.yml

@@ -4,9 +4,6 @@ on:
   push:
     branches:
       - main          # still run tests for main
-    tags:
-      - 'v*'          # run tests for version tags (e.g. v0.9.1)
-
 jobs:
   test-unit:
     uses: ./.github/workflows/test-unit.yml