[GitHub] add stale action

[bmarwell]

Adds a stale-action so old issues and PRs get a stale label after 90 days and get closed after further 7 days when no-one commented.

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[lprimak]

90 days? Maybe a year is more appropriate? I am not quite sure.. but leave it up to you :)

[bmarwell]

90 days? Maybe a year is more appropriate? I am not quite sure.. but leave it up to you :)

The default is 60 days! If we are constantly re-opening issues, I will happily change it! :)