Add and apply Naming/BlockForwarding rubocop rule

This syntax has been added in Ruby 3.1.

Not using a variable name might not be very descriptive, but it's just
as descriptive as using "block" as a variable name. Using just `&` we
get the same amount of information than using `&block`: that we're
passing a block.

We're still using `&action` in `around_action` methods because here we
aren't using a generic name for the variable, so (at least for now) we
aren't running this cop on controllers using `around_action`.

spec/sessions_helper.rb

@@ -1,3 +1,3 @@
-def in_browser(name, &block)
-  Capybara.using_session(name, &block)
+def in_browser(name, &)
+  Capybara.using_session(name, &)
 end