build(hooks): update Commitizen to ^4.2.4

An indirect dependency of Commitizen (`merge`) is currently failing the
NPM.js auditor due to vulnerability CVE-2020-28499. This commit moves
the minimum version of Commitizen to 4.2.4, which has resolved this
vulnerability.

Change-Id: Ia9455bdbe02f7406c1a106f173c4095943a201ed
Signed-off-by: Chris Kay <chris.kay@arm.com>

package.json

@@ -6,7 +6,7 @@
   "devDependencies": {
     "@commitlint/cli": "^11.0.0",
     "@commitlint/config-conventional": "^11.0.0",
-    "commitizen": "^4.2.2",
+    "commitizen": "^4.2.4",
     "cz-conventional-changelog": "^3.3.0",
     "husky": "^5.0.4"
   }