Dashboard Conversion: Set panel level datasource when converting from v2 to v1 (#115777)

* set panel level ds when converting from v2 to v1

* remove comment

* lint

* always set ds in sqr

* improve

* Apply suggestions from code review

Co-authored-by: Ivan Ortega Alba <ivanortegaalba@gmail.com>

---------

Co-authored-by: Ivan Ortega Alba <ivanortegaalba@gmail.com>

apps/dashboard/pkg/migration/conversion/testdata/output/v2beta1.ds-data-query.v1beta1.json

@@ -150,6 +150,9 @@
     "liveNow": false,
     "panels": [
       {
+        "datasource": {
+          "type": "prometheus"
+        },
         "fieldConfig": {
           "defaults": {
             "color": {
@@ -245,6 +248,10 @@
         "type": "timeseries"
       },
       {
+        "datasource": {
+          "type": "prometheus",
+          "uid": "gdev-prometheus"
+        },
         "fieldConfig": {
           "defaults": {
             "color": {
@@ -341,6 +348,9 @@
         "type": "timeseries"
       },
       {
+        "datasource": {
+          "type": "grafana-testdata-datasource"
+        },
         "fieldConfig": {
           "defaults": {
             "color": {
@@ -400,6 +410,10 @@
         "type": "stat"
       },
       {
+        "datasource": {
+          "type": "grafana-testdata-datasource",
+          "uid": "gdev-testdata"
+        },
         "fieldConfig": {
           "defaults": {
             "color": {