fix: update examples to consume independent shenyu-client-java artifacts

[lll-peanut]

Closes #6371

Replace all ${project.version} references for shenyu-client-* and shenyu-spring-boot-starter-client-* dependencies with centralized ${shenyu.client.java.version} property in
parent POM.

Changes

• Parent POM: Added <shenyu.client.java.version>2.7.0.3</shenyu.client.java.version> and <dependencyManagement> entries for all 9 client artifacts
• 21 pom.xml files: Removed <version>${project.version}</version> from 19 client dependency declarations — all now inherit version from parent
• README.md: Fixed Verification section — replaced broken directory-level mvn spring-boot:run with reactor-root mvn spring-boot:run -pl <module>

Why

Examples now consume released shenyu-client-java artifacts from Maven Central, decoupling from the main repo's source modules — matching the recommended approach for real users.

Make sure that:

• You have read the contribution guidelines.
• You submit test cases (unit or integration tests) that back your changes.
• Your local test passed ./mvnw clean install -Dmaven.javadoc.skip=true.