Add support for Rails 8.0

We needed to:

* Bring over quite a few things from `rails app:update`, which hopefully
  should make future upgrades easier,
* Fix a change to the `enum` `ActiveModel` signature, which changed
  between Rails 6.0 and 7.0,
* Add a version check from 7.0 for `raise_on_missing_callback_actions`

https://rubyonrails.org/2024/11/7/rails-8-no-paas-required

Closes #2703

.github/workflows/main.yml

@@ -46,7 +46,7 @@ jobs:
       fail-fast: false
       matrix:
         ruby: [3.1, 3.2, 3.3]
-        appraisal: [rails61, rails70, pundit21]
+        appraisal: [rails61, rails70, rails80, pundit21]
         include:
           - ruby: '3.0'
             appraisal: rails60

Appraisals

@@ -11,6 +11,10 @@ appraise "rails70" do
   gem "rails", "~> 7.0"
 end
 
+appraise "rails80" do
+  gem "rails", "~> 8.0"
+end
+
 appraise "pundit21" do
   gem "pundit", "~> 2.1.0"
 end