Update references to RFC 2616

Replace references to the old RFC 2616 (HTTP 1.1) with references
to the new RFCs 7230 to 7235.

This commit also deprecates:
 - HttpStatus.USE_PROXY
 - HttpStatus.REQUEST_ENTITY_TOO_LARGE in favor of HttpStatus.PAYLOAD_TOO_LARGE
 - HttpStatus.REQUEST_URI_TOO_LONG in favor of HttpStatus.URI_TOO_LONG

Issue: SPR-12067