chore: drop Rails 6 support (doesn't work well with Sqlite3/json columns)

Author: did

.github/workflows/verify.yml

@@ -20,7 +20,7 @@ jobs:
     strategy:
       matrix:
         node: [20, 23]
-        gemfile: ["Gemfile.rails_6", "Gemfile.rails_7_0", "Gemfile.rails_7_2", "Gemfile"]
+        gemfile: ["Gemfile.rails_7_0", "Gemfile.rails_7_2", "Gemfile"]
 
     steps:
       - name: Checkout code

Gemfile.lock

@@ -5,7 +5,7 @@ PATH
       jbuilder (>= 2, < 3)
       kaminari (~> 1.2.1)
       maglev-injectable (~> 2.1.1)
-      rails (>= 6, < 9)
+      rails (>= 7, < 9)
       vite_rails (>= 3, < 4)
       vite_ruby (>= 3.5)