Enable nohttp-checkstyle

[Stephan202]

Suggested commit message:

Enable `nohttp-checkstyle` (#206)

While this Checkstyle configuration only flags `http://` usages in
Maven-managed source files (thus not in e.g. `pom.xml` or `README.md`
files), this is a low-effort improvement.

[Stephan202]

Also filed spring-io/nohttp#56.

[rickie]

Interesting check, let's do it.

1. Should we also adopt this internally?
2. In the commit message we often mention check (when we turn on a Checkstyle check or module), but in this case it is already quite clear, so also fine with the current proposal.

[Stephan202]

W.r.t. (1): would be good to do so. First step would be to assess impact (how much to clean up, how many edge cases).

W.r.t. (2): Fair! I went with the current proposal because we're also pulling in a whole new dependency 🤷