Use File.exist? instead of File.exists?

We've noticed the following warning while testing the upgrade to
Ruby 3.0:

warning: File.exists? is deprecated; use File.exist? instead

We're adding a Rubocop rule so we don't call the deprecated method
in the future.

.rubocop.yml

@@ -164,6 +164,9 @@ Lint/BooleanSymbol:
 Lint/ConstantDefinitionInBlock:
   Enabled: true
 
+Lint/DeprecatedClassMethods:
+  Enabled: true
+
 Lint/DuplicateBranch:
   Enabled: true