Apply a label to issues that are tracked by a milestone

[surchs]

To get a quick sense of what is tied to the current milestone, it'd be nice if issues that are tracked by a milestone get a label.

e.g. "from Milestone" or something similar

[github-actions]

We want to keep our issues up to date and active. This issue hasn't seen any activity in the last 30 days.
We have applied the _flag:stale label to indicate that this issue should be reviewed again.
When you review, please reread the spec and then apply one of these three options:

• prioritize: apply the flag:schedule label to suggest moving this issue into the backlog now
• close: if the issue is no longer relevant, explain why (give others a chance to reply) and then close.
• archive: sometimes an issue has important information or ideas but we won't work on it soon. In this case
  apply the someday label to show that this won't be prioritized. The stalebot will ignore issues with this
  label in the future. Use sparingly!

[surchs]

Unlikely to work since "Tracked by" is not exposed via the API yet: https://github.com/orgs/community/discussions/39106