Add shared workflow to flag stale issues

[kenoir]

In order to reduce cognitive overhead for developers and make it clearer what Pull Requests need dealing with we could mark old PRs as stale and close them if they are not going to be worked on.

See this conversation for context: https://wellcome.slack.com/archives/C3TQSF63C/p1705328381576849

We could appreciatively borrow https://github.com/guardian/.github/blob/main/workflow-templates/stale.yml, and investigate whether we can add this for all repositories at the organisation level.

[kenoir]

Closing as done.