doc(redmine6): add guide for using ApplicationRecord, refs #pbi-7854

Author: CLAGOR

docs/Major_releases/v15.md

@@ -16,11 +16,11 @@ for developers and maintainers who need to understand the implications of these
 
 ### Major Framework Upgrades
 
-- ruby `">= 3.3.1"` → `">= 3.3.7", "< 3.4.0"`
+- ruby `>= 3.3.1` → `~> 3.4.0`
   - Added upper bound to Ruby version
-- rails `"~> 6.1.7.10"` → `"~> 7.2.2.1"`
+- rails `~> 6.1.7.10` → `7.2.2.1`
   - Major framework upgrade
-- rack `"~> 2.2.3"` → `"~> 3.1.3"`
+- rack `~> 2.2.3` → `>= 3.1.3`
 
 ### Breaking Changes
 
@@ -190,6 +190,24 @@ import "../src/easy_legacy_js/context_menu";
 - Move legacy JavaScript files to `app/frontend/src/easy_legacy_js/`
 - Import them in `app/frontend/entrypoints/easy_legacy_js.js`
 
+#### 2. Inheriting ActiveRecord models from ApplicationRecord
+
+All ActiveRecord models should now inherit from `ApplicationRecord` instead of `ActiveRecord::Base`.
+
+**Before:**
+```ruby
+class User < ActiveRecord::Base
+  # ...
+end
+```
+
+**After:**
+```ruby
+class User < ApplicationRecord
+  # ...
+end
+```
+
 ### Recommendations for Upgrading
 
 1. **Test thoroughly** - The framework changes are substantial and require extensive testing