Bump redmine from 5 to 6 in /tests/redmine

Bumps redmine from 5 to 6.

---
updated-dependencies:
- dependency-name: redmine
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

tests/redmine/Dockerfile

@@ -1,5 +1,5 @@
 # checkov:skip=CKV_DOCKER_2:Ensure that HEALTHCHECK instructions have been added to container images
-FROM redmine:5
+FROM redmine:6
 
 # enable the rest API
 RUN sed -i "/rest_api_enabled:/{n;s/^  default: 0/  default: 1/;}" /usr/src/redmine/config/settings.yml