Make server.port conditionally required on HTTP server semconv

[trask]

Part of #385

Changes

Makes server.port conditionally required on HTTP server semconv when it is not the default for the given url.scheme (80/443).

This avoids the question of whether missing server.port means default or unknown.

And we expect server.port to be available to all HTTP server instrumentation.

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• CHANGELOG.md updated for non-trivial changes.
• schema-next.yaml updated with changes to existing conventions.