gradle: bump org.openapi.generator from 7.8.0 to 7.9.0 (#204)

Bumps org.openapi.generator from 7.8.0 to 7.9.0.

---
updated-dependencies:
- dependency-name: org.openapi.generator
  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>

build.gradle

@@ -9,7 +9,7 @@ buildscript {
 }
 
 plugins {
-  id 'org.openapi.generator' version '7.8.0'
+  id 'org.openapi.generator' version '7.9.0'
   id 'java-library'
   id 'maven-publish'
   id 'net.researchgate.release' version '3.0.2'