Add stale bot

[gsmendoza]

This is part of an org-wide project to add a stale bot to every repo of solidusio.

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

• I have written a thorough PR description.
• I have kept my commits small and atomic.
• I have used clear, explanatory commit messages.

The following are not always needed (cross them out if they are not):

• [ ] I have added automated tests to cover my changes.
• [ ] I have attached screenshots to demo visual changes.
• [ ] I have opened a PR to update the guides.
• [ ] I have updated the readme to account for my changes.

[mergify]

It looks like this PR is missing a label to determine the type of change it introduces. The maintainer should add one of the following labels:

• bug for bugfixes.
• enhancement for new features and improvements.
• documentation for documentation changes.
• security for security patches.
• removed for feature removals.
• infrastructure for internal changes that should not go in the changelog.

Additionally, the maintainer may also want to add one of the following:

• breaking for breaking changes.
• deprecated for feature deprecations.

Once the correct labels have been set, simply remove the needs changelog label label from this PR so I can merge it.