refactor(platform/gitlab): consolidate duplicate code

[RahulGautamSingh]

Changes

• Create a function out of the duplicate code from fetchPrList and getPr
• Merge the logic of massagePr into util fn prInfo

Note: While the modification have caused changes to both code & tests (snaps to be accurate). Looking closely at the changes in snaps once can see that they are not result actual logical changes. But only trimming of unwanted fields present in the snaps. ie. they are not important to the test logic

Context

Reduce duplicate code and also use correct types.

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[viceice]

see comment

[renovate-release]

🎉 This PR is included in version 39.25.0 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)
• 39.25.0

Your semantic-release bot 📦🚀