Use stable branch for --edge option when possible

This alleviates the need to update the code when there is a new stable
branch (for example, as done in #41454).

railties/lib/rails/generators/app_base.rb

@@ -276,8 +276,9 @@ def rails_gemfile_entry
             GemfileEntry.path("rails", Rails::Generators::RAILS_DEV_PATH)
           ]
         elsif options.edge?
+          edge_branch = Rails.gem_version.prerelease? ? "main" : [*Rails.gem_version.segments.first(2), "stable"].join("-")
           [
-            GemfileEntry.github("rails", "rails/rails", "main")
+            GemfileEntry.github("rails", "rails/rails", edge_branch)
           ]
         elsif options.main?
           [

railties/test/generators/app_generator_test.rb

@@ -863,8 +863,20 @@ def test_dev_option
   end
 
   def test_edge_option
-    generator([destination_root], edge: true, skip_webpack_install: true)
-    run_generator_instance
+    Rails.stub(:gem_version, Gem::Version.new("2.1.0")) do
+      generator([destination_root], edge: true, skip_webpack_install: true)
+      run_generator_instance
+    end
+
+    assert_equal 1, @bundle_commands.count("install")
+    assert_file "Gemfile", %r{^gem\s+["']rails["'],\s+github:\s+["']#{Regexp.escape("rails/rails")}["'],\s+branch:\s+["']2-1-stable["']$}
+  end
+
+  def test_edge_option_during_alpha
+    Rails.stub(:gem_version, Gem::Version.new("2.1.0.alpha")) do
+      generator([destination_root], edge: true, skip_webpack_install: true)
+      run_generator_instance
+    end
 
     assert_equal 1, @bundle_commands.count("install")
     assert_file "Gemfile", %r{^gem\s+["']rails["'],\s+github:\s+["']#{Regexp.escape("rails/rails")}["'],\s+branch:\s+["']main["']$}

railties/test/generators/plugin_generator_test.rb

@@ -250,8 +250,20 @@ def test_dev_option
   end
 
   def test_edge_option
-    generator([destination_root], edge: true)
-    run_generator_instance
+    Rails.stub(:gem_version, Gem::Version.new("2.1.0")) do
+      generator([destination_root], edge: true)
+      run_generator_instance
+    end
+
+    assert_empty @bundle_commands
+    assert_file "Gemfile", %r{^gem\s+["']rails["'],\s+github:\s+["']#{Regexp.escape("rails/rails")}["'],\s+branch:\s+["']2-1-stable["']$}
+  end
+
+  def test_edge_option_during_alpha
+    Rails.stub(:gem_version, Gem::Version.new("2.1.0.alpha")) do
+      generator([destination_root], edge: true)
+      run_generator_instance
+    end
 
     assert_empty @bundle_commands
     assert_file "Gemfile", %r{^gem\s+["']rails["'],\s+github:\s+["']#{Regexp.escape("rails/rails")}["'],\s+branch:\s+["']main["']$}