Add time-based triage workflows for stale issues

Add a cron-based workflow (e.g. like [this one](https://github.com/github/super-linter/blob/e0790b48355071d1952a237dccecc83db6763f32/.github/workflows/stale.yml)), which checks if an issue is in the "Need info" column on the [Issue triage project board](https://github.com/vector-im/element-web/projects/27). If it is, and it's not been actioned in 21 days, then move it to the "Follow up" column.

Then if the issue is in "Follow up" column and has been commented on in the last 21 days, then move it to the "Followed up" column.

Then if the issue is in "Followed up" column and hasn't been actioned in 21 days, move it to the "Candidate for closing" column.

@ankur12-1610 would you be interested in this one as well?