Adding a new 2.0.39-beta version for SDK release for Beta for Mail changes

[maigovannon]

New branch for a so-called 2.0.39-beta branch which will track all the Beta releases for the Mail SDK changes

NOTE: This change won't be merged into main but it is only to create a PR for review.

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Release Notes:

This release introduces a major feature to the App Engine Mail API: the ability to send emails via an
external SMTP service. This provides a more flexible and robust way to send emails from your App Engine
application.

Mail API: SMTP Support

Key Features:

• SMTP Mail Sending: Implemented the ability to send emails through a configured SMTP server. This
  functionality can be enabled by setting the USE_SMTP_MAIL_SERVICE environment variable.
• Admin Email Support: AdminEmailMessage is now supported when using the SMTP transport. Recipients for
  admin emails are configured via the ADMIN_EMAIL_RECIPIENTS environment variable.
• Error Handling & Resilience: Added specific error handling for common SMTP issues, such as authentication
  and connection errors, which are now surfaced as standard Mail API exceptions.
• Comprehensive Tests: A full suite of unit tests has been added to ensure the reliability and correctness
  of the new SMTP sending functionality under various conditions.

[ludoch]

Not ready, see https://github.com/GoogleCloudPlatform/appengine-java-standard/actions/runs/17233070319

[maigovannon]

CI on push is successful: https://github.com/GoogleCloudPlatform/appengine-java-standard/actions/runs/17233061855. The failures for the actions on the pull_request are because this branch is behind the HEAD since we wanted to create a release only from the latest SDK changes.

The pull_request is made to run on main branch which will fail here: https://github.com/GoogleCloudPlatform/appengine-java-standard/blob/main/.github/workflows/maven.yml#L21-L23

[ludoch]

In order to to a mvn release, we need this to be 2.0.39-beta-SNAPSHOT.
The release tooling will do the right thing removing SNAPSHOT