Mark HTTP semantic conventions as Feature-freeze

[trask]

Prior to marking the HTTP semantic conventions as Stable, we are planning to mark is at Frozen.

The only purpose of the Frozen period is to give us time to migrate a handful of HTTP instrumentations (across at least 3 different languages) to the frozen HTTP semantic conventions, so we can be more comfortable before marking them Stable.

Languages and libraries:

• Java (@mateuszrzeszutek and @trask)
  • Tomcat HTTP server
  • Netty HTTP server (low-level networking library)
  • Java 11 HTTP client
  • OkHttp HTTP client
  • Netty and Netty Reactor HTTP client

TODO add a similar list for at least 2 other languages