Update k8s-agent go 1.26.1

[oleksandrsan]

Changes

1. Update Go to 1.26.1
2. Opt out of the new WatchList protocol which is default in client-go 0.35.

• Context: client-go v0.35 enables the WatchListClient feature gate by default, which changes how the
  reflector initializes informers. It now expects watchers to implement the WatchList
  protocol (sending Added events followed by a Bookmark).
• Issue: The PodMetrics informer uses a custom polling-based watch shim (metricsWatch) because the
  metrics API doesn't support native watching. This shim doesn't implement the WatchList
  protocol, causing the reflector to enter a retry loop where it repeatedly creates and
  stops watchers, never syncing. The repeated Stop() calls also triggered a close of closed
  channel panic.
• Fix (also mentioned in istio client):
  • Opt the PodMetrics ListWatch out of WatchList semantics via
    IsWatchListSemanticsUnSupported(), so the reflector falls back to the classic list+watch
    flow.
  • Guard metricsWatch.Stop() with sync.Once to prevent double-close panics.

Testing

✅ Lint passed
✅ Tests passed with race detector (go test ./... -race)