chore: exemption requested PRs should not be marked as stale (aws#30873)

PRs with the exemption requested label should not be marked as stale since the onus is on the maintainers to address the exemption before the contributor can satisfy our linter.

Unlikely to be abused, but we can reconsider this approach if it is.

.github/workflows/close-stale-prs.yml

@@ -25,5 +25,5 @@ jobs:
           important-checks-regex: AutoBuildv2Project1C6BFA3F
           warn-message: This PR has been in the STATE state for 3 weeks, and looks abandoned. To keep this PR from being closed, please continue work on it. If not, it will automatically be closed in a week.
           close-message: This PR has been deemed to be abandoned, and will be automatically closed. Please create a new PR for these changes if you think this decision has been made in error.
-          skip-labels: contribution/core,pr-linter/do-not-close,pr/needs-maintainer-review
+          skip-labels: contribution/core,pr-linter/do-not-close,pr/needs-maintainer-review,pr-linter/exemption-requested
           close-label: closed-for-staleness