Stale GHA instructions to reopen issue do not work for non GitHub team members

[tkknight]

🐛 Bug Report

How To Reproduce

Steps to reproduce the behaviour:

1. The stale GitHub action suggests to mention @SciTools/iris-devs to have a stale issue re-opened, however we have learned a mention can only be done by someone already in the GitHub team.

Expected behaviour

The stale comments should state a way that works.

Environment

N/A

Additional context

A GitHub action (GHA) could be created to look for specific text such as /not stale or ping @Scitools/iris-devs on a new issue comment that will then add a comment itself and mention the team, I think the GHA will have permission to do this (needs testing).

conda-forge does something similar as per the comment in #4617.

Alternatively a GHA could be triggered to just reopen the issue, similar to the above but triggred by /reopen.

[bjlittle]

Also see #4133, which is another use case where we want non-members of SciTools to be able to easily @mention us to get our attention

[wjbenfold]

This is also assumed to work in the developer guide here: https://scitools-iris.readthedocs.io/en/stable/developers_guide/contributing_documentation_easy.html

[tkknight]

Mentioneing a group is also not allowed in a GitHub Action unless you create a personal access token according to this: https://github.saobby.my.eu.orgmunity/t/team-mention-from-bot-not-working/144323

[github-actions]

In order to maintain a backlog of relevant issues, we automatically label them as stale after 500 days of inactivity.

If this issue is still important to you, then please comment on this issue and the stale label will be removed.

Otherwise this issue will be automatically closed in 28 days time.

[trexfeathers]

Discussion in @SciTools/peloton: a full solution to the tagging problem involving GHA is over-engineered. Modifying the comment to NOT suggest tagging seems like a good enough solution.