chore(deps): update dependency pngjs to v4 (#447)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [pngjs](https://github.com/lukeapage/pngjs2) | devDependencies | major | [`^3.3.3` -> `^4.0.0`](https://renovatebot.com/diffs/npm/pngjs/3.4.0/4.0.0) |

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packages/google-privacy-dlp/samples/package.json

@@ -24,7 +24,7 @@
     "chai": "^4.2.0",
     "mocha": "^7.0.0",
     "pixelmatch": "^5.0.0",
-    "pngjs": "^3.3.3",
+    "pngjs": "^4.0.0",
     "uuid": "^7.0.0"
   }
 }