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`,
* Make a few changes to CI (on top of #2712) to adjust the Ruby versions
  we run CI against.

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

Closes #2703

.github/workflows/main.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [3.1, 3.2, 3.3]
+        ruby: [3.2, 3.3]
     env:
       PGHOST: localhost
       PGUSER: administrate
@@ -65,10 +65,13 @@ 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
+        exclude:
+          - ruby: 3.1
+            appraisal: rails80
     env:
       PGHOST: localhost
       PGUSER: administrate

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