Tidy openapi-generator runner tags for jobs on devops-general runner

Summary

Since migrating the jobs in openapi-generator.gitlab-ci.yml to the GKE devops-general runner, we have observed failures in the openapi:schema job due to the Docker daemon not being available.

This issue tracks a review of all jobs in the openapi-generator.gitlab-ci.yml template to test their compatibility with the devops-general runner and apply any necessary adjustments to ensure they execute successfully.

Upon further investigation, the failing jobs were specific to the activate-account/api repository, not the openapi jobs in this template repository. Therefore, this issue is being refocused on tidying up the runner tags in the openapi template to avoid repeatedly copying the gke-devops-general string and to keep the template more DRY.