ci: replace Sendgrid usage in Test Notify on Failure GitHub workflow

[dlachaume]

Content

This PR updates the Test Notify on Failure GitHub workflow to replace Sendgrid with Mailgun for sending failure notifications.

Instead of using the peter-evans/sendgrid-action@v1 action with Sendgrid, it now uses dawidd6/action-send-mail@v6 with Mailgun as the SMTP provider.

Pre-submit checklist

• Branch
  • Tests are provided (if possible)
  • Crates versions are updated (if relevant)
  • CHANGELOG file is updated (if relevant)
  • Commit sequence broadly makes sense
  • Key commits have useful messages
• PR
  • All check jobs of the CI have succeeded
  • Self-reviewed the diff
  • Useful pull request description
  • Reviewer requested
• Documentation
  • Update README file (if relevant)
  • Update documentation website (if relevant)
  • Add dev blog post (if relevant)
  • Add ADR blog post or Dev ADR entry (if relevant)
  • No new TODOs introduced

Issue(s)

Closes #2617

[github-actions]

Test Results

    3 files  ±0    105 suites  ±0   17m 52s ⏱️ +27s
2 071 tests ±0  2 071 ✅ ±0  0 💤 ±0  0 ❌ ±0 
4 285 runs  ±0  4 285 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 7859f59. ± Comparison against base commit 55ca841.

[jpraynaud]

LGTM 👍