http: change DEP0066 to a runtime deprecation

Change doc-only deprecation for _headers and _headerNames accessors to a
runtime deprecation.

PR-URL: https://github.com/nodejs/node/pull/24167


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>