Fix stale Javadoc references in api/incubator

[thswlsqls]

Fixes #8614

Description

Three public Javadoc comments in :api:incubator point at things that moved or no longer exist.

• InstrumentationConfigUtil.getOrNull(): drop "Returns null if ConfigProvider#getInstrumentationConfig() is null" — make getInstrumentationConfig non-null #7954 (a46b073) made that method non-null and deleted the null check, leaving the Javadoc behind. That return path cannot happen; the other two null conditions are real and stay.
• ConfigProvider.getInstrumentationConfig(): retarget the schema link from schema/instrumentation.json (404) to schema/instrumentation.yaml — Yaml source of truth for JSON schema files opentelemetry-configuration#411 deleted the JSON files and made YAML the source of truth.
• ExtendedContextPropagators: retarget the README link from contrib extended-tracer/README.md (404) to the api/incubator README section that replaced it — remove extended tracer, which has been merged into the main repo opentelemetry-java-contrib#1129 removed that module after the code moved here.
• Related: Remove link to 404ing curio-server-framework #8605, which removed a 404ing link that had no replacement.

Testing done

• Docs-only, test-exempt: no signature change, so no apidiff entry; no user-facing behavior change, so no CHANGELOG entry.
• ./gradlew :api:incubator:check — 96 tests passed.
• Verified with curl: both old links 404, both new links 200.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.63%. Comparing base (63ebd94) to head (c66e9fd).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8615   +/-   ##
=========================================
  Coverage     91.63%   91.63%           
  Complexity    10326    10326           
=========================================
  Files          1013     1013           
  Lines         27295    27295           
  Branches       3205     3205           
=========================================
  Hits          25011    25011           
  Misses         1558     1558           
  Partials        726      726           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.