Avoid autolinking YouTrack issue numbers followed by letters

What does this MR do?

This MR prevents auto-linking of YouTrack issues when when number contains letters. I want to make this change as sometimes YouTrack integration links Sentry issues. Sentry issues are base-36 strings. This doesn't fully solve the issue, as there is a possibility a Sentry issue will contain digits, but it noticeably decreases the probability of Sentry issue being accidentally linked as if it was a YouTrack issue.

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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