fix: generated model classes don't have generic lombok Setter annotation

[manusa]

Description

Fixes #6100

Add specific setAdditionalProperties setter method instead.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• Feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change
• Chore (non-breaking change which doesn't affect codebase;
  test, version modification, documentation, etc.)

Checklist

• Code contributed by me aligns with current project license: Apache 2.0
• I Added CHANGELOG entry regarding this change
• I have implemented unit tests to cover my changes
• I have added/updated the javadocs and other documentation accordingly
• No new bugs, code smells, etc. in SonarCloud report
• I tested my code in Kubernetes
• I tested my code in OpenShift

[shawkins]

LGTM if the goal is to eventually remove lombok, then we have to take on this responsibility

[sonarcloud]

Quality Gate failed

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)

See analysis details on SonarCloud

[manusa]

the goal is to eventually remove lombok, then we have to take on this responsibility

The goal is to replace this generator with something simpler (see #6100 (comment)).
I've got something mostly working now, but I still prefer to do some easy simplifications to the legacy model generator beforehand (mostly those that involve generating invalid or redundant code -such as this one-).