Add deprecation for hsts

[rocketeerbkw]

Routes could be configured to return the HSTS header by using the hsts field. The format was:

environments:
  main:
    routes:
      - nginx:
        - "www.example.com":
            hsts: max-age=31536000

HSTS is still supported, but the field names have changed, so existing hsts fields should throw a deprecation.

[smlx]

The linter has a deprecated profile for this purpose (it can already check for monitoring URLs).

See #14 (comment)

The same goes for #22 and #23.