Allow optional angle brackets in (Envelope-To) mail header

What does this MR do?

Allow forwarded emails to be successfully processed in the service desk.

When #37359 (closed) was solved by !22354 (merged) we overlooked some small thing.
My Envelope-To header had some angle brackets around the mail address.

Example:

Envelope-To: <incoming+gitlabhq/gitlabhq+auth_token@appmail.example.com>

But the test only covers the case when there a no brackets around the mail address.

Enhancing the regular expression that checks the format with optional brackets fixes this problem.

First commit enhances the example with mentioned brackets, second contains my attempt to fix this.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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