Bump helmet from 5.1.1 to 6.0.0

Bumps [helmet](https://github.com/helmetjs/helmet) from 5.1.1 to 6.0.0.
- [Release notes](https://github.com/helmetjs/helmet/releases)
- [Changelog](https://github.com/helmetjs/helmet/blob/main/CHANGELOG.md)
- [Commits](helmetjs/helmet@v5.1.1...v6.0.0)

---
updated-dependencies:
- dependency-name: helmet
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

package.json

@@ -119,7 +119,7 @@
     "express-validator": "^6.14.2",
     "govuk-elements-sass": "^3.1.3",
     "govuk-frontend": "^4.3.1",
-    "helmet": "^5.1.1",
+    "helmet": "^6.0.0",
     "http-errors": "^2.0.0",
     "http-status-codes": "^2.2.0",
     "indefinite": "^2.4.1",