build(deps): bump org.openapi.generator from 7.20.0 to 7.21.0

Bumps org.openapi.generator from 7.20.0 to 7.21.0.

---
updated-dependencies:
- dependency-name: org.openapi.generator
  dependency-version: 7.21.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

gradle/libs.versions.toml

@@ -33,7 +33,7 @@ ignite="2.17.0"
 apacheCxf="3.6.10"
 node="24.14.0"
 modelixBuildtools="2.0.1"
-openapi = "7.20.0"
+openapi = "7.21.0"
 micrometer = "1.16.4"
 detekt = "1.23.8"
 xmlunit = "2.11.0"