etcdserver: increase warnApplyDuration from 10ms to 100ms

When running test suites for a client locally I'm getting spammed by log
lines such as:

    etcdserver: apply entries took too long [14.226771ms for 1 entries]

The comments in #6278 mention there were future plans of increasing the
threshold for logging these warnings, but it hadn't been done yet.

etcdserver/apply.go

@@ -35,7 +35,7 @@ const (
 	// to apply functions instead of a valid txn ID.
 	noTxn = -1
 
-	warnApplyDuration = 10 * time.Millisecond
+	warnApplyDuration = 100 * time.Millisecond
 )
 
 type applyResult struct {