Spring Security Add default value for version in gitHubCheckMilestoneHasNoOpenIssues task

The gitHubCheckMilestoneHasNoOpenIssues Gradle task can check if a given milestone has any open issues associated with it. Currently it looks at the version provided in the nextVersion parameter. We should make the nextVersion parameter optional and as a default look up the next version using the gitHubNextReleaseMilestone Gradle task.

Related to gh-9693 and gh-10451