refactor: reduce branchConcurrentLimit to 2

Prior to this change 8 branches where being processed concurrently. This can cause renovate to run slow on big repositories where there are a lot of branches.

Author: alan-agius4

renovate-presets/default.json5

@@ -8,6 +8,7 @@
   // Schedule Renovate to run during off-peak hours
   schedule: ['after 10:00pm every weekday', 'before 5:00am every weekday', 'every weekend'],
   prConcurrentLimit: 8,
+  branchConcurrentLimit: 2,
   prHourlyLimit: 4,
   timezone: 'America/Tijuana',