Test & fix with helmet v7 and set as a peer dependency on `^6||^7`

[PaulThompson]

In order to pull in the most recent version of helmet, test updating to Helmet latest (v7).

In order to allow further updates to Helmet in future set as a peer dependency. Set peer dependency for ^7 for current and tested Helmet v7. Set to peer dependency || ^6 to back-compatibly support Helmet v6 in case that is needed.

The fix for Helmet v7 required removal of helmet.expectCt()

From https://github.com/helmetjs/helmet/blob/main/CHANGELOG.md:

Expect-CT is no longer part of Helmet. If you still need it, you can use the expect-ct package.
https://github.com/helmetjs/helmet/issues/378