ngalert openapi: Use same basePath as rest of Grafana (#79025)

* ngalert openapi: Use same `basePath` as rest of Grafana
Currently, there are two issues that prevent easily merging `ngalert` and grafana openapi specs:
- The basePath is different. `grafana` has `/api` and `ngalert` has `/api/v1`. I changed `ngalert` to use `/api`
- The `ngalert` endpoints have their basePath in the each operation path. The basePath should actually be omitted
---------

Co-authored-by: Yuriy Tseretyan <yuriy.tseretyan@grafana.com>

pkg/services/ngalert/api/authorization_test.go

@@ -47,6 +47,7 @@ func TestAuthorize(t *testing.T) {
 
 	t.Run("should not panic on known routes", func(t *testing.T) {
 		for path, methods := range paths {
+			path := swaggerSpec.Spec().BasePath + path
 			for _, method := range methods {
 				require.NotPanics(t, func() {
 					api.authorize(method, path)