Allow single letter external issue IDs

What does this MR do?

Modify the regular expression that is used to link external issues on comments and MR titles.

What are the relevant issue numbers?

Closes #52803 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• [-] Conforms to the style guides
• [-] Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.