fix(file source): Execute checkpoint migration for every new file for kubernetes_log source.

[Rajin9601]

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fix https://github.com/vectordotdev/vector/issues/20798

Checkpoint migration was only executed when file_server is being initialized. However kubernetes_log's PathProvider doesn't load paths at the start-up (because it have to fetch k8s metadata). So for kubernetes_log, migration will not be executed.

This PR suggests executing checkpoint migration whenever a new file appears. Migration process is quite simple, so I don't think it's too computationally intensive.

[bits-bot]

All committers have signed the CLA.

[Rajin9601]

@jszwedko sure! changelog entry is added here. https://github.com/vectordotdev/vector/pull/21231/commits/a224bec72c82ca05615613dc7f9344876c224f6b

[jszwedko]

Perfect, thanks @Rajin9601 !

[jszwedko]

Just a note that make check-clippy is failing on this PR.