chore: remove notifications experiment (#14869)

Notifications have proved stable in the [mainline release of
v2.15](https://github.com/coder/coder/releases/tag/v2.15.0), and in
preparation for v2.16 we're moving this to stable.

enterprise/coderd/coderd.go

@@ -448,7 +448,6 @@ func New(ctx context.Context, options *Options) (_ *API, err error) {
 		// with the below route, we need to register this route without any mounts or groups to make both work.
 		r.With(
 			apiKeyMiddleware,
-			httpmw.RequireExperiment(api.AGPL.Experiments, codersdk.ExperimentNotifications),
 			httpmw.ExtractNotificationTemplateParam(options.Database),
 		).Put("/notifications/templates/{notification_template}/method", api.updateNotificationTemplateMethod)
 	})

enterprise/coderd/notifications_test.go

@@ -23,7 +23,6 @@ func createOpts(t *testing.T) *coderdenttest.Options {
 	t.Helper()
 
 	dt := coderdtest.DeploymentValues(t)
-	dt.Experiments = []string{string(codersdk.ExperimentNotifications)}
 	return &coderdenttest.Options{
 		Options: &coderdtest.Options{
 			DeploymentValues: dt,