workflows/labels: lower API calls reservoir to 500

First data shows, that we're unlikely to need more than 250 within an
hour of regular activity. Once this is empty, we'll need to wait until
the next hourly refill - thus, we'll rather set this a bit higher to be
on the safe side.

The hourly limit is at 5000 and we peaked around 3500, so far. We'll
certainly have to look into reducing API calls, but this should still
work out for now.

(cherry picked from commit 8e1f869261)

.github/workflows/labels.yml

@@ -72,8 +72,8 @@ jobs:
               maxConcurrent: 1,
               // Hourly limit is at 5000, but other jobs need some, too!
               // https://docs.github.com/en/rest/using-the-rest-api/rate-limits-for-the-rest-api
-              reservoir: 1000,
-              reservoirRefreshAmount: 1000,
+              reservoir: 500,
+              reservoirRefreshAmount: 500,
               reservoirRefreshInterval: 60 * 60 * 1000
             })
             // Pause between mutative requests