Align common.schema.json with Entity type

Added the field `targetRef` to match the Typescript type

Signed-off-by: Christian Probst <christian.probst.dk@gmail.com>

.changeset/shaggy-crabs-return.md

@@ -0,0 +1,5 @@
+---
+'@backstage/catalog-model': patch
+---
+
+Added targetRef to common.schema.json to match the Typescript type

packages/catalog-model/src/schema/shared/common.schema.json

@@ -43,6 +43,11 @@
         },
         "target": {
           "$ref": "#reference"
+        },
+        "targetRef": {
+          "type": "string",
+          "minLength": 1,
+          "description": "The entity ref of the target of this relation."
         }
       }
     },