fix(core): add parallelism to target defaults schema (#27710)

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## Current Behavior
Parallelism is part of the project schema but not `targetDefaults`. This
means it's highlighted as a warning in `nx.json`

## Expected Behavior
There should be no warning.

packages/nx/schemas/nx-schema.json

@@ -506,6 +506,11 @@
             "type": "object"
           }
         },
+        "parallelism": {
+          "type": "boolean",
+          "default": true,
+          "description": "Whether this target can be run in parallel with other tasks"
+        },
         "inputs": {
           "$ref": "#/definitions/inputs"
         },