re-use transport for AdminInfo() call (#14571)

avoids creating new transport for each `isServerResolvable`
request, instead re-use the available global transport and do
not try to forcibly close connections to avoid TIME_WAIT
build upon large clusters.

Never use httpClient.CloseIdleConnections() since that can have
a drastic effect on existing connections on the transport pool.

Remove it everywhere.

internal/event/target/webhook.go

@@ -228,8 +228,6 @@ func (target *WebhookTarget) Send(eventKey string) error {
 
 // Close - does nothing and available for interface compatibility.
 func (target *WebhookTarget) Close() error {
-	// Close idle connection with "keep-alive" states
-	target.httpClient.CloseIdleConnections()
 	return nil
 }