Add "implements" to the default issue closing message regex

What does this MR do?

Allows closing an issue by using "implements" in commit message, as suggested by @stavros in #38424 (closed). All the following words are allowed: Implement, Implements, Implemented, Implementing, implement, implements, implemented, implementing.

Are there points in the code the reviewer needs to double check?

You know... regex is always easy to mess things up.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #38424 (closed)