feat: add validation check for manual bp release notes

[alicelovescake]

This PR:

• fixes Validate Manual Backport Release Notes #281
• Add new helper updateManualBackportReleaseNotes
• Updates tests and fixtures

[dsanders11]

I think this is going to get clobbered by the update a bit further down in this file:

trop/src/index.ts

Lines 358 to 362 in 47f78d4

	await updateBackportValidityCheck(context, checkRun, {
	title: 'Valid Backport',
	summary: `This PR is declared as backporting "#${oldPRNumber}" which is a valid PR that has been merged into ${pr.base.repo.default_branch}`,
	conclusion: CheckRunStatus.SUCCESS,
	});

Might take some more refactoring to ensure the clobbering doesn't happen. However, I'm a bit worried about a regression since we don't have good test coverage here. Could you look at expanding test coverage in spec/index.spec.ts to cover updateBackportValidityCheck? A separate PR first expanding test coverage might be best since it will keep the changes in this one more concise.

[alicelovescake]

Thanks for the pointers!

• Created a PR to expand test coverage of index.spec.ts test: add test for updateBackportValidityCheck #289
• Updated the logic in this PR to move the release notes check after all the failure map check so it doesn't get clobbered
• After the test PR gets merged, I will update this PR with additional updateBackportValidityCheck so we can be confident it works