internal/lsp/source: enable the new diff with allExperiments

When enabling all experiments (done by VS Code nightly), switch to the
checked version of the new diff implementation.

Also remove some experimental settings that are now on by default.

Updates golang/go#52967

Change-Id: Id272c4a646006a739e49d48f0f09b2f8b0982bab
Reviewed-on: https://go-review.googlesource.com/c/tools/+/419981
gopls-CI: kokoro <noreply+kokoro@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Peter Weinberger <pjw@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

internal/lsp/source/options.go

@@ -802,10 +802,9 @@ func (o *Options) AddStaticcheckAnalyzer(a *analysis.Analyzer, enabled bool, sev
 // should be enabled in enableAllExperimentMaps.
 func (o *Options) EnableAllExperiments() {
 	o.SemanticTokens = true
-	o.ExperimentalPostfixCompletions = true
 	o.ExperimentalUseInvalidMetadata = true
 	o.ExperimentalWatchedFileDelay = 50 * time.Millisecond
-	o.SymbolMatcher = SymbolFastFuzzy
+	o.NewDiff = "checked"
 }
 
 func (o *Options) enableAllExperimentMaps() {