fix: datasource in dashboard.json (#19426)

Other panels have properly set `"uid": "$datasource"`, but for some reason, this doesn't.
Let's fix it.

Signed-off-by: TomasR <linux@rohlik.xyz>

examples/dashboard.json

@@ -4014,7 +4014,7 @@
       "collapsed": true,
       "datasource": {
         "type": "prometheus",
-        "uid": "prometheus"
+        "uid": "$datasource"
       },
       "gridPos": {
         "h": 1,
@@ -4031,7 +4031,7 @@
           "dashes": false,
           "datasource": {
             "type": "prometheus",
-            "uid": "prometheus"
+            "uid": "$datasource"
           },
           "fieldConfig": {
             "defaults": {
@@ -4078,7 +4078,7 @@
             {
               "datasource": {
                 "type": "prometheus",
-                "uid": "prometheus"
+                "uid": "$datasource"
               },
               "expr": "sum(increase(argocd_redis_request_total{namespace=~\"$namespace\"}[$interval])) by (failed)",
               "refId": "A"
@@ -4119,7 +4119,7 @@
         {
           "datasource": {
             "type": "prometheus",
-            "uid": "prometheus"
+            "uid": "$datasource"
           },
           "refId": "A"
         }
@@ -4426,4 +4426,4 @@
   "uid": "LCAgc9rWz",
   "version": 2,
   "weekStart": ""
-}
+}