Ignore incoming emails with X-Autoreply

What does this MR do?

Ignore incoming emails with X-Autoreply

Some messages may not contains the RFC-compliant Auto-Submitted header, instead they may use X-Autoreply header for automated replies (e.g. out of office emails).

Note that this fix may not be definitive, some blogs and other "unofficial" sources mention also other headers to check, but we can add other iteratively if needed. Redmine uses same set (https://www.redmine.org/projects/redmine/repository/entry/branches/4.0-stable/app/models/mail_handler.rb#L82) so let's use the same for now.

Related to #30327 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team