#24698 Update apache http client to newer version

to close https://github.com/OpenAPITools/openapi-generator/pull/24697
to close https://github.com/OpenAPITools/openapi-generator/pull/24696

PR checklist

• Read the contribution guidelines.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

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Summary by cubic

Unifies the Apache HttpClient version used by the Java apache-httpclient generator to 5.6.3 to pick up upstream fixes and align Gradle and Maven outputs. Previously Gradle templates/samples used 5.1.3 and Maven used 5.2.1; now both use 5.6.3.

• Updates httpclient_version in build.gradle.mustache and <httpclient-version> in pom.mustache; regenerates affected samples.
• No generator logic changes; only dependency version bump. Generated clients will now reference org.apache.httpcomponents.client5:httpclient5 5.6.3.
• Migration: Regenerate Java clients to adopt 5.6.3. If you pin the dependency in your project, no action is required.