Update split SDK to 6.0.2 to fix sync bug (#17060)

* Update split SDK to 6.0.2 to fix sync bug

* Vendor and tidy

vendor/github.com/splitio/go-toolkit/v4/backoff/backoff.go

@@ -0,0 +1,35 @@
+package backoff
+
+import (
+	"math"
+	"sync/atomic"
+	"time"
+)
+
+// Interface is the backoff interface
+type Interface interface {
+	Next() time.Duration
+	Reset()
+}
+
+// Impl implements the Backoff interface
+type Impl struct {
+	base    int64
+	current int64
+}
+
+// Next returns how long to wait and updates the current count
+func (b *Impl) Next() time.Duration {
+	current := atomic.AddInt64(&b.current, 1)
+	return time.Duration(math.Pow(float64(b.base), float64(current))) * time.Second
+}
+
+// Reset sets the current count to 0
+func (b *Impl) Reset() {
+	atomic.StoreInt64(&b.current, 0)
+}
+
+// New creates a new Backoffer
+func New() *Impl {
+	return &Impl{base: 2}
+}