Back-compat is old clients talking to new daemons, not the other way
around. New clients should always use the V2 lazy diff path — there's
no need to fall back to V1 based on a feature flag.
- Remove useLazyDiff feature gate from git-diff-pane
- Always use useCheckoutDiffMetadataQuery (V2)
- Remove useCheckoutDiffQuery (V1) usage from the component
- Remove supportsLazyDiffRpcs utility (no longer needed client-side)
- Server still advertises the flag and serves V1 for old clients