Update show_exceptions options in system tests

We were getting a deprecation warning after upgrading to Rails 7.1:

```
DEPRECATION WARNING: Setting action_dispatch.show_exceptions to true is
deprecated. Set to :all instead.
```

So we're passing `:all`.

I've checked whether we can remove the `:show_exceptions` tag completely
now that the new default option is `:rescuable`. The answer is "no" ;).
Removing the `:show_exceptions` tag makes the tests fail.

spec/spec_helper.rb

@@ -102,7 +102,7 @@ RSpec.configure do |config|
 
     allow(Rails.application).to receive(:env_config) do
       config.merge(
-        "action_dispatch.show_exceptions" => true,
+        "action_dispatch.show_exceptions" => :all,
         "action_dispatch.show_detailed_exceptions" => false,
         "consider_all_requests_local" => false
       )