Use alternative to Sendgrid for email notifications in CI

[jpraynaud]

Why

We want to replace Sendgrid which free tier is being sunset with another SMTP server provider with a free tier in the CI workflows.

What

Replace the SMTP provider in the CI workflows.

How

• Select and use a standard SMTP server GitHub action (action-send-mail or see)
• Select a SMTP provider with a free tier:
  • https://www.mailgun.com
  • https://www.mailjet.com
  • IOHK?
• Setup the SMTP provider:
  • Create an account (@jpraynaud)
  • Setup DNS for mithril.network (@jpraynaud)