Add script to create TW milestone plan issues

To automate the monthly TW milestone plan issue, see #512 (comment 780861203).

• Enable CI/CD
• Add GITLAB_API_PRIVATE_TOKEN in the project's CI/CD variables (used by the gitlab gem). Use a project access token with developer access (we need at least developer to get the upcoming milestone).
• Test that script works in CI
  • CI job https://gitlab.com/axil/playground/-/jobs/1903639995
  • Issue created: axil/playground#11
• Add a schedule to create the issue on the 15th of each month. Cron job: 0 0 15 * * => https://gitlab.com/gitlab-org/technical-writing/-/pipeline_schedules/192695/edit

Improvements:

• Figure out the milestone automatically and adding this to the title/description.
• Assign the team to the issue.
  • This can actually be done in the template itself!