Document the user mailing list deprecation better

[oleg-nenashev]

Proposal

There is a WireMock user mailing list, but you are unlikely to discover it unless you try to create an issue or see it in the documentation. I( think we should keep the mailing list along Slack, but it needs proper documentation. As @tomakehurst mentioned in wiremock/.github#2, this mailing list is about being deprecated and archived. So we should cleanup references

Reference any relevant documentation, other materials or issues/pull requests that can be used for inspiration.

• https://groups.google.com/g/wiremock-user