Make `not_planned` the default close-issue-reason

[danielcompton]

The problem

Currently, everyone who uses actions/stale will need to know to configure the close-issue-reason setting to not_planned to get the new close type - https://github.blog/changelog/2022-05-19-the-new-github-issues-may-19th-update/

The solution

Make the default value 'not_planned' instead of ''. The reason matches the use in stale - "Won't fix, canceled, duplicate, stale"

[github-actions]

This issue is stale because it has been open 30 days with no activity. Remove stale label or comment or this will be closed in 5 days

[davorpa]

bump

It have a current PR fullfilling requirements #807

[luketomlinson]

Released in v6: https://github.com/actions/stale/releases/tag/v6.0.0