chore: Update stale.yml to only apply to pulls, not issues (#200)

Since we're using GH issues to track work (thanks @misskniss and @tjcran for setting this up!) - we probably don't want the stale bot to close untouched issues every month 😬 

I considered using a separate stale-duration (like, 365 days for issues) - but just turned it off now. Easy for us to change if we want to adjust the process.

.github/stale.yml

@@ -2,6 +2,8 @@
 daysUntilStale: 14
 # Number of days of inactivity before a stale issue is closed
 daysUntilClose: 5
+# Only apply the stale logic to pulls, since we are using issues to manage work
+only: pulls
 # Label to apply when stale.
 staleLabel: stale
 # Comment to post when marking an issue as stale. Set to `false` to disable