Bump dawidd6/action-send-mail from 15 to 16 (#11)

Author: dependabot[bot]

.github/workflows/buildtest.yml

@@ -153,7 +153,7 @@ jobs:
     if: github.repository_owner == 'moves-rwth' && always()
     steps:
       - uses: technote-space/workflow-conclusion-action@v3
-      - uses: dawidd6/action-send-mail@v15
+      - uses: dawidd6/action-send-mail@v16
         with:
           server_address: ${{ secrets.STORM_CI_MAIL_SERVER }}
           server_port: 587