Issue description

There are several tasks in the grails-core build that makes calls to github to list the tags for the grails-core repo. Now that the repos are merged, we should consider using the git commands instead so we aren't affected by throttling & so that locally we don't have to have a GITHUB_TOKEN

This will also be used to update Spring Security Plugin which has it's own copy of CreateReleaseDropDownTask
https://github.com/apache/grails-spring-security/blob/7320e1bc98d68de167d6e4732173e7f41a30eb8c/buildSrc/src/main/groovy/grails/plugin/springsecurity/CreateReleaseDropDownTask.groovy#L56