feat(cache): file cache cleanup

[rarkins]

Changes

Checks file cache for expired items at the end of a run.

Context

Closes #13732

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[rarkins]

I'll add tests once gotten positive feedback on the implementation and whether it should be an opt-in flag at first. I think it seems safe enough seeing as it's wrapped in a try/catch

[viceice]

can be very time consuming for very big caches

[rarkins]

It's at the end of the run and it needs to be cleaned up sometime

[rarkins]

can be very time consuming for very big caches

I think it needs to be done though. And it cleans at the end of a Renovate run.

Worst case:

• It times out or they kill it manually
• It will have cleaned out X entries by that time
• It will have less to do next time

We're also releasing it in a major so if it somehow makes it impossible to use for some, they can hold off upgrading and raise an issue (or just wipe their cache)

[viceice]

something wrong with your merge. too many changes here

[rarkins]

Will add tests