Update dawidd6/action-send-mail action to v21 (#27376)

Signed-off-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>
Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

.github/workflows/announce-lts-rc.yml

@@ -17,7 +17,7 @@ jobs:
           discourse-author-username: jenkins-release-bot
           discourse-category: 23
       - name: Post on mailing list
-        uses: dawidd6/action-send-mail@d35571df1982970871070b5e02f01d3b6095cb74 # v19
+        uses: dawidd6/action-send-mail@03a02a49409134de07636216fb379a026c52991e # v21
         with:
           server_address: smtp.gmail.com
           server_port: 465