kvserver: enable kv.rangefeed.buffered_sender.enabled by default

This patch enables kv.rangefeed.buffered_sender.enabled by default. We've gained
confidence in the feature through scale testing and enabling it metamorphically
since v25.2. We also currently don't believe enabling this feature would cause
any performance regressions or negative impact.

Epic: none
Release note: none

Author: wenyihu6

pkg/kv/kvserver/replica_rangefeed.go

@@ -90,7 +90,7 @@ var RangefeedUseBufferedSender = settings.RegisterBoolSetting(
 	"kv.rangefeed.buffered_sender.enabled",
 	"use buffered sender for all range feeds instead of buffering events "+
 		"separately per client per range",
-	metamorphic.ConstantWithTestBool("kv.rangefeed.buffered_sender.enabled", false),
+	metamorphic.ConstantWithTestBool("kv.rangefeed.buffered_sender.enabled", true),
 )
 
 func init() {