ci: don't specify sqlite3 version

sqlite3 >= 2.5.0 doesn't work with Ruby 3.1.

Author: kou

Gemfile

@@ -21,9 +21,10 @@ end
 
 group :activerecord do
   gem "activerecord"
-  # This must be synchronized with `gem "sqlite", "..."` in
+  # We may need to specify version explicitly to align with `gem
+  # "sqlite", "..."` in
   # lib/active_record/connection_adapters/sqlite3_adapter.rb.
-  gem "sqlite3", ">= 2.1"
+  gem "sqlite3"
 end
 
 group :cruby do