Address PR #1985 feedback: use safer git add

Use `git add -u docs-mslearn/` instead of wildcard pattern to only
stage tracked files that were actually modified by the workflow.

🤖 Generated with [Claude Code](https://claude.ai/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: flanakin

.github/workflows/update-mslearn-dates.yml

@@ -64,6 +64,6 @@ jobs:
         run: |
           git config --local user.email "github-actions[bot]@users.noreply.github.com"
           git config --local user.name "github-actions[bot]"
-          git add docs-mslearn/**/*.md
+          git add -u docs-mslearn/
           git commit -m "chore: Update ms.date in docs-mslearn files"
           git push