Draft: Align Draft Merge Request Regex Behaviour

What does this MR do?

The front-end regex that marks an MR as draft (in issuable_form.js) does not match the back-end regex (in regex.rb), and the docs were also inconsistent with behaviour.

This MR attempts to align all three in their marking of an MR as draft and updates the specs for both MRs and commits, with the goal of matching as closely as possible the functionality as-currently-documented.

Closes #238967 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• N/A Database guides
• N/A 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
• N/A 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:

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