kvflowcontrol: set kvadmission.flow_control.mode default to apply_to_all

With testing now in place, this commit changes the default cluster
setting value of `kvadmission.flow_control.mode` from
`apply_to_elastic`, to `apply_to_all`.

Now by default, regular work will be subject to replication admission
control, whereby a quorum will be allowed to proceed, queuing entries
to any non-quorum required replica, when send tokens are exhausted.

For more details, see cockroachdb#123509.

Resolves: cockroachdb#133838
Release note (performance improvement): Regular writes are now subject
to admission control by default, meaning that non-quorum required
replicas may not be told about new writes from the leader if they are
unable to keep up. This brings a large performance improvement during
instances where there is a large backlog of replication work towards a
subset of node(s), such as node restarts. The setting can be reverted to
the <=24.3 default by setting `kvadmission.flow_control.mode` to
`apply_to_elastic`.

pkg/kv/kvserver/kvflowcontrol/kvflowcontrol.go

@@ -44,8 +44,8 @@ var Mode = settings.RegisterEnumSetting(
 	"determines the 'mode' of flow control we use for replication traffic in KV, if enabled",
 	metamorphic.ConstantWithTestChoice(
 		"kvadmission.flow_control.mode",
-		modeDict[ApplyToElastic], /* default value */
-		modeDict[ApplyToAll],     /* other value */
+		modeDict[ApplyToAll],     /* default value */
+		modeDict[ApplyToElastic], /* other value */
 	),
 	modeDict,
 )