Fix Turbo.config.forms.confirm deprecation message

Turbo 8 deprecated `Turbo.setConfirmMethod()` and instead moved it to `Turbo.config.forms.confirm` instead (see hotwired/turbo#1216). Because of that, avo is throwing deprecation messages in the javascript console:

    Please replace `Turbo.setConfirmMethod(confirmMethod)` with `Turbo.config.forms.confirm = confirmMethod`. The top-level function is deprecated and will be removed in a future version of Turbo.

I've updated the code to use the new config object instead, which should resolve the deprecation warning.

app/javascript/js/custom-confirm.js

@@ -1,6 +1,6 @@
 import { Turbo } from '@hotwired/turbo-rails'
 
-Turbo.setConfirmMethod((message) => {
+Turbo.config.forms.confirm = (message) => {
   const dialog = document.getElementById('turbo-confirm')
   dialog.querySelector('p').textContent = message
   dialog.showModal()
@@ -16,4 +16,4 @@ Turbo.setConfirmMethod((message) => {
       resolve(dialog.returnValue === 'confirm')
     }, { once: true })
   })
-})
+}