build(deps): bump org.openapi.generator from 7.20.0 to 7.21.0 (#343)

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>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Patrick Boos <patrick.boos@getyourguide.com>

Author: dependabot[bot]

gradle/libs.versions.toml

@@ -2,7 +2,7 @@
 java = "21"
 spring-boot = "4.0.5"
 spring-dependency-management = "1.1.7"
-openapi-generator = "7.20.0"
+openapi-generator = "7.21.0"
 openapi-tools = "0.2.9"
 swagger = "2.2.46"
 swagger-request-validator = "2.46.1"