DataSource: Default data source is no longer a persisted state but just the default data source for new panels (#45132)

* PanelEdit: Change the meaning of default data source to be just that the default for new panels

* Added migration, and also migration for annotation datasource prop to data source refs

* fix

* Fixing tests

* Fixes to annotation

* Fixing unit test

public/app/features/templating/template_srv.ts

@@ -103,7 +103,7 @@ export class TemplateSrv implements BaseTemplateSrv {
     for (const variable of this.getAdHocVariables()) {
       const variableUid = variable.datasource?.uid;
 
-      if (variableUid === ds.uid || (variable.datasource == null && ds?.isDefault)) {
+      if (variableUid === ds.uid) {
         filters = filters.concat(variable.filters);
       } else if (variableUid?.indexOf('$') === 0) {
         if (this.replace(variableUid) === datasourceName) {