Update stale.yml

[lucacome]

Proposed changes

Problem: The stale workflow is closing issues with an epic label. These are issues that are supposed to be open for a long time so we don't want to mark them as stale after 14 days

Solution: Add 'epic' to the list of exempt labels

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes,
please add a brief note that summarizes the change.

NONE