When Che 7.y.0 releases are done, send reminder to TLs to clean milestones of any outstanding unresolved issues

[nickboldt]

We need some GH automation to ping TLs via mattermost that will tell them:

• link to current milestone, eg., https://github.com/eclipse/che/milestone/121 (4 unresolved issues) or https://github.com/eclipse/che/milestone/122 (10 unresolved issues)
• instructions re: their options:

• resolve issue, or
• move to backlog for no longer scheduled work, or
• move to next milestone for WIP

Once that's in place, we could also enable GH automation to automatically close milestones when they are devoid of unresolved issues.

[nickboldt]

Related to this topic, it seems that the 7.19 and 7.20 milestones still have unresolved issues in them, so we could also send nag-reminders at weekly intervals until the milestones are properly cleaned up.

Best case: sending mail to individuals, so they know it's an action for them directly
Worst case: sending mail to che-dev list, shaming people into proper behaviour

[nickboldt]

WIP PR for this eclipse-che/che-release#16

[nickboldt]

to send MM notifications, see https://github.com/eclipse/che-release/actions/workflows/release-send-mattermost-announcement.yml or #18906