internal/lsp/source: switch the default diff algorithm to "checked"

We've been using this setting in experimental for some time without
issue, so switch it to default.

For golang/go#52967

Change-Id: Ib4d786e689d4b0f009195cc86d7dd5d8269cf424
Reviewed-on: https://go-review.googlesource.com/c/tools/+/427534
Reviewed-by: Peter Weinberger <pjw@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

gopls/internal/lsp/source/options.go

@@ -165,6 +165,7 @@ func DefaultOptions() *Options {
 				CompletionDocumentation: true,
 				DeepCompletion:          true,
 				ChattyDiagnostics:       true,
+				NewDiff:                 "both",
 			},
 			Hooks: Hooks{
 				// TODO(adonovan): switch to new diff.Strings implementation.
@@ -822,7 +823,6 @@ func (o *Options) EnableAllExperiments() {
 	o.SemanticTokens = true
 	o.ExperimentalUseInvalidMetadata = true
 	o.ExperimentalWatchedFileDelay = 50 * time.Millisecond
-	o.NewDiff = "checked"
 }
 
 func (o *Options) enableAllExperimentMaps() {