feat(onboarding): create OnboardingCache object

[RahulGautamSingh]

Changes

• Use the onboardingCache object created in feat(onboarding): use cache to check if repo is onboarded #20733 to store onboardingBranch related info like: isModified and isConflicted

Context

• Preview changes to other files in onboarding branches #20797 (comment)
• It will reduce the number of git operations performed when processing existing onboardingPrs

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

[RahulGautamSingh]

Not necessary anymore as the onboardingCache in #20893 satisfies all the requirements.