ci: stale_issue: Use actions/stale@v8

This commit updates the stale issue workflow to use the stale action
v8, which is based on node.js 16 and @actions/core 1.10.0, in
preparation for the upcoming removal of the deprecated GitHub features.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>
(cherry picked from commit f03190a2)