Show the first four eligible PRs by maturity rather than submission

[peternewman]

Due to retrospective tagging in non-topic repos, submission is not always equal to maturity.

Description

As per the title.

Test process

None so far, need to add an array check to the spec to ensure the result is sorted correctly.

Requirements to merge

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes