Bump dawidd6/action-send-mail from 3 to 4

Bumps [dawidd6/action-send-mail](https://github.com/dawidd6/action-send-mail) from 3 to 4.
- [Release notes](https://github.com/dawidd6/action-send-mail/releases)
- [Commits](dawidd6/action-send-mail@v3...v4)

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

Signed-off-by: dependabot[bot] <[email protected]>

.github/workflows/buildtest.yml

@@ -139,7 +139,7 @@ jobs:
     if: github.repository_owner == 'moves-rwth' && always()
     steps:
       - uses: technote-space/workflow-conclusion-action@v3
-      - uses: dawidd6/action-send-mail@v3
+      - uses: dawidd6/action-send-mail@v4
         with:
           server_address: ${{ secrets.STORM_CI_MAIL_SERVER }}
           server_port: 587