Default feature flag for cat-file cache to enabled

Now that we've monitored the cache in production with a good performance gain and no noticeable errors, we can default the feature flag to enabled. (note #1664 (comment 172213843))

Once it's been enabled by default for a full release in self-hosted environments, we can remove the feature flag

Proposal

Remove the feature flag
Backport feature flag removal to %11.11

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