Add package engines (#1840)

* Add "engines.npm" to package.json

* Add "engines.node" to package.json files

* Update engines in docs/package-lock.json

* Add patch changeset for engines

Author: jclem

.changeset/itchy-yaks-care.md

@@ -0,0 +1,5 @@
+---
+'@primer/react': patch
+---
+
+Set Node.js and npm versions to ">=12" and ">=7", respectively, in package.json manifests, and update package-lock.json files accordingly.