Include monthly tasks template in new issue URL

Why is this change being made?

With this, the monthly maintenance doc will link to a new issue with a pre-filled title and already using the right template:

https://gitlab.com/gitlab-org/technical-writing/-/issues/new?issue[title]=Docs%20project%20maintenance%20tasks%2C%20Month%20YYYY&issuable_template=tw-monthly-tasks

This will also gently enforce a consistent format for titling these issues. Gently because anyone who uses the template manually will still have to fill the title themselves.

More: https://docs.gitlab.com/ee/user/project/issues/managing_issues.html#using-a-url-with-prefilled-values

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

---