feat(dashboard): on demand config migration

[RahulGautamSingh]

Changes

• modify config migration logic to handle the requirements made in linked issue
• add tests for the same

Context

Closes: #19783

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

Test Runs

Config Migration: false: checkbox-added, migration pr linked in dd, checkbox-added-when-closed-pr-found, migration-pr-linked (closed-pr-exists)

Config Migration true: pr created & linked no checkbox needed, checkbox-added-if-closed-pr-found, pr-linked-again-after-checkbox-checked (closed-pr-exists)

Modified Migration PR

[rarkins]

Needs docs. Potentially even a new page to discuss config migration in general which can then be linked from the configMigration docs entry

[rarkins]

Also try to use a consistent term in logging so that you can Ctrl-F to search for all applicable messages. e.g. configMigration: or "Config migration" in each message

[HonkingGoose]

Lots of suggested changes, to summarize:

• Add headings
• Rewrite to focus on the reader
• Explain a bit more about why we want/need config migration
• Rewrite the user-facing texts from Renovate about config migration (dashboard text, PR link text, etc.)

[rarkins]

Can be merged after manual testing from @zharinov

[zharinov]

It worked on my test repo

[renovate-release]

🎉 This PR is included in version 38.126.0 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)
• 38.126.0

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