Bump dawidd6/action-send-mail from 15 to 16

Bumps [dawidd6/action-send-mail](https://github.com/dawidd6/action-send-mail) from 15 to 16.
- [Release notes](https://github.com/dawidd6/action-send-mail/releases)
- [Commits](dawidd6/action-send-mail@v15...v16)

---
updated-dependencies:
- dependency-name: dawidd6/action-send-mail
  dependency-version: '16'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.github/workflows/buildtest.yml

@@ -358,7 +358,7 @@ 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