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

<!-- Please make sure you have read the submission guidelines before
posting an PR -->
<!--
https:/nrwl/nx/blob/master/CONTRIBUTING.md#-submitting-a-pr
-->

<!-- Please make sure that your commit message follows our format -->
<!-- Example: `fix(nx): must begin with lowercase` -->

<!-- If this is a particularly complex change or feature addition, you
can request a dedicated Nx release for this pull request branch. Mention
someone from the Nx team or the `@nrwl/nx-pipelines-reviewers` and they
will confirm if the PR warrants its own release for testing purposes,
and generate it for you if appropriate. -->

## 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.

(cherry picked from commit 4abcf61)

packages/nx/schemas/nx-schema.json

@@ -499,6 +499,11 @@
  "type": "object"
  }
  },
+ "parallelism": {
+ "type": "boolean",
+ "default": true,
+ "description": "Whether this target can be run in parallel with other tasks"
+ },
  "inputs": {
  "$ref": "#/definitions/inputs"
  },