feat: add stale app yaml

[CecileRobertMichon]

Reason for Change:

Adds stale app configuration yaml, a GitHub App built with Probot that closes abandoned Issues and Pull Requests after a period of inactivity.

Issue Fixed:

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Notes:

[mboersma]

/lgtm

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: CecileRobertMichon, mboersma

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[codecov]

Codecov Report

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The diff coverage is n/a.

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