ci: make sure to exit on maitenance milestone failure (#7656)

**Related Issue:** #7531

## Summary

The action correctly determined that it was a maintenance milestone and
[set the error
message](https://github.com/Esri/calcite-design-system/actions/runs/5988880275/job/16244580547#step:3:39).
However, it looks like
[`core.setFailed()`](https://github.com/actions/toolkit/tree/main/packages/core#exit-codes)
sets the exit code but doesn't actually exit. And then the else block
with `process.exit(0)` overrode the exit code in the next iteration.
Explicitly exiting will fix the issue.

.github/scripts/limitPullRequestsForMaintenanceReleases.js

@@ -51,6 +51,7 @@ module.exports = async ({ github, context, core }) => {
           allowedLabels,
         )}.`,
       );
+      process.exit(1)
     } else {
       core.notice(
         "The current milestone is not for a Maintenance release, ending run.",