Upgrade helmet version to add support for `Feature-Policy`
The `Feature-Policy` header is currently not supported in helmet version `3.12.0` as used by this library.
See:
https://helmetjs.github.io/docs/feature-policy/

This first became available in `helmet@3.14.0` as discussed in [this issue](https://github.com/helmetjs/helmet/issues/173)

I am aware that the way the dependency is specified, it theoretically allows for automatic update without a change to the `package.json` file of this library. We can however improve the situation for developers that heavily rely on `package-lock.json` files that potentially fix the dependency on a lower version.