Fix stale Javadoc references in api/incubator

[thswlsqls]

Is your feature request related to a problem? Please describe.
Three public Javadoc comments in api/incubator describe things that no longer exist:

• InstrumentationConfigUtil.getOrNull() (config/InstrumentationConfigUtil.java line 96) documents "Returns null if ConfigProvider#getInstrumentationConfig() is null". make getInstrumentationConfig non-null #7954 made that method non-null and deleted the null check, but left the Javadoc. That return path cannot happen.
• ConfigProvider.getInstrumentationConfig() (config/ConfigProvider.java line 23) links schema/instrumentation.json, which 404s. Yaml source of truth for JSON schema files opentelemetry-configuration#411 deleted the JSON schema files and made YAML the source of truth.
• ExtendedContextPropagators (propagation/ExtendedContextPropagators.java line 21) links opentelemetry-java-contrib/extended-tracer/README.md, which 404s. remove extended tracer, which has been merged into the main repo opentelemetry-java-contrib#1129 removed that module after the code moved into this repo.

Describe the solution you'd like
Drop the impossible null condition, and point the two links at the surviving targets: schema/instrumentation.yaml and the api/incubator README section that replaced the contrib README. Javadoc only — no signature or behavior change.

Describe alternatives you've considered
Removing the ExtendedContextPropagators sentence, as #8605 did for a link with no replacement. Here a replacement exists, so retargeting keeps the information.

Spec alignment
N/A — documentation only.

Additional context

• Affected module: :api:incubator
• Artifact stability: alpha
• Backward compatible: yes