workflow: add stale PR last update setter

This patch adds a Python script, that uses GitHub's GraphQL
API to provide automatic last update date for PR items on
any specified GitHub ProjectsV2 board.

Since there is no versatile enough  way to automate a
GitHub ProjectV2 by its own means, that is done via
standard mechanism of on schedule workflow, which runs
the script every hour.

NO_DOC=workflow
NO_TEST=workflow
NO_CHANGELOG=workflow