Exclude generated OpenAPI models from SonarCloud duplication (CPD) check

Context:
The generated model classes under com/adyen/model/<api>/** follow a
repetitive OpenAPI Generator template (getters/setters, equals/hashCode/
toString, isSet* flags, getExplicitNulls, toJson/fromJson). Sonar's
Copy-Paste Detection flags huge numbers of duplicated blocks across
these files, e.g. CI run: https://github.com/Adyen/adyen-java-api-library/actions/runs/29031747289/job/86166267497?pr=2002
logged 'Too many duplication references ... Keep only the first 100
references' for hundreds of blocks, all within com/adyen/model/**. This
duplication is a byproduct of code generation, not a real maintenance
risk, and drowns out genuine duplication signal in hand-written code.

Change:
Add sonar.cpd.exclusions=**/com/adyen/model/*/** to pom.xml, using the
same glob already applied to sonar.coverage.exclusions for consistency.

This only removes generated model packages from the duplication metric;
Sonar still analyzes them for bugs, vulnerabilities, and code smells,
and hand-written code (serializer packages, services, httpclient, etc.)
remains fully covered by duplication detection.