HTTP semantic convention stability countdown

[trask]

Opening this as a planning and tracking issue to coordinate the last few items before we can mark HTTP semantic conventions as stable.

Most important:

• Review, approve, merge build-tools PR: Update sampling-relevant MUST -> SHOULD build-tools#230
• Make new release from build-tools repository

Then finish reviewing, approving, merging these PRs:

• Change sampling relevant from MUST to SHOULD (and update build-tools version) #486
• Make user_agent.original and http.request.header.* attributes sampling relevant on HTTP server spans #467
• Change network.protocol.name from opt-in to conditionally required #478
• Remove outdated http.request.header.host guidance #479

And finally:

• Mark HTTP semantic conventions as stable #377

[arminru]

Done! See #377 and https://github.com/open-telemetry/semantic-conventions/releases/tag/v1.23.0.