rubocop: set Exclude keys to merge

When we added an AllCops.Exclude key in c673609 (rubocop: ignore
db/migrate/, 2022-10-05), that meant we stopped respecting the built-in
config. And that config contains vendor/**, among other things.

This wasn't a problem for me locally, because my bundled gems are found
elsewhere. But in our CI environment, they are in vendor/, which is what
caused all of the problems with out-dated rubocop config in gems.
Hopefully this fixes it.

.rubocop.yml

@@ -2,6 +2,10 @@ require:
   - rubocop-rails
   - rubocop-performance
 
+inherit_mode:
+  merge:
+    - Exclude
+
 AllCops:
   TargetRubyVersion: 3.1
   Exclude: