[Tests] pin jest-diff/pretty-format to dodge npm-alias install break on node 4/5

`pretty-format@30.4.0` (released 2026-05-07) added `npm:` alias entries to
its `dependencies` (`react-is-18`, `react-is-19`). The npm 5.x that ships
with node 4 and 5 — used by the `node-minors.yml` matrix — does not
recognise the `alias` dependency type and aborts the bare `npm install`
with `npm ERR! Invalid dependency type requested: alias` before the
`after_install` step ever runs.

Pulled into the tree via:

    eslint-plugin-react -> eslint-doc-generator@^3.0.2
                        -> jest-diff@^30.2.0  (floats up to 30.4.x)
                        -> pretty-format@<exact same version>

Two-pronged pin so both old and new npm installs avoid 30.4.x:

- `devDependencies.jest-diff: 30.3.0` — direct pin so npm 5/6 hoists
  jest-diff@30.3.0 to the top level; eslint-doc-generator's `^30.2.0`
  range is satisfied by it, so no nested 30.4.x is fetched.
- `overrides.pretty-format: 30.3.0` — belt-and-braces for npm >= 8.3
  (the eslint-9+ matrix and any node-minors entry on a newer npm).

`30.3.0` is the latest pretty-format release without `npm:` aliases.

Author: captaindonald

package.json

@@ -80,6 +80,7 @@
     "glob": "=10.3.7",
     "istanbul": "^0.4.5",
     "jackspeak": "=2.1.1",
+    "jest-diff": "30.3.0",
     "ls-engines": "^0.8.1",
     "markdownlint-cli": "^0.8.0 || ^0.32.2",
     "mocha": "^5.2.0",
@@ -91,6 +92,9 @@
   "peerDependencies": {
     "eslint": "^3 || ^4 || ^5 || ^6 || ^7 || ^8 || ^9.7"
   },
+  "overrides": {
+    "pretty-format": "30.3.0"
+  },
   "engines": {
     "node": ">=4"
   },