Fix javadoc examples in context propagation interfaces - #8715

Description

• The class javadoc examples on TextMapPropagator and ContextPropagators use pre-1.0 API names, so copying them does not compile.
• Setter/Getter no longer exist: the types are TextMapSetter<C>/TextMapGetter<C>, and the setter method is set, not put.
• The server example implemented only get (with no return) and omitted the required keys; it now mirrors ExtendedContextPropagators.TEXT_MAP_GETTER.
• Context.Key, Context.key(...), ContextKey.get(context) and context.withValue(...) become ContextKey.named(...), context.get(key) and context.with(key, value).
• MyConcernPropagator was missing the class keyword, used an undeclared concern, and did not implement the abstract fields().
• HttpTraceContext was removed in 1.0; the composite example now uses W3CTraceContextPropagator.getInstance().
• Incomplete fix of Javadoc code snippets use methods that don't exist #4140, which touched this same ContextPropagators block but left Setter/Getter/HttpTraceContext behind.

Testing done

• Docs-only, test-exempt: ./gradlew :context:check passed, and docs/apidiffs/current_vs_latest/*.txt is unchanged.
• Compiled the corrected examples with javac against :context and :api:all — every type and signature resolves.
• ./gradlew :context:javadoc --rerun-tasks — BUILD SUCCESSFUL, no invalid-tag in the generated HTML.