Upgrade to Ruby 3.4, Bundler 4, fix frozen string literal warnings

.ruby-version

@@ -0,0 +1 @@
+3.4.4

Gemfile.lock

@@ -1,7 +1,7 @@
 PATH
   remote: .
   specs:
-    lamby (6.0.1)
+    lamby (7.0.0)
       lambda-console-ruby
       rack (>= 3.0.0)
 
@@ -245,6 +245,7 @@ PLATFORMS
   arm64-darwin-21
   arm64-darwin-22
   arm64-darwin-23
+  arm64-darwin-25
   x86_64-linux
 
 DEPENDENCIES
@@ -263,4 +264,4 @@ DEPENDENCIES
   webrick
 
 BUNDLED WITH
-   2.3.26
+  4.0.13

lib/lamby/handler.rb

@@ -38,7 +38,7 @@ def set_cookies
     end
 
     def body
-      @rbody ||= ''.tap do |rbody|
+      @rbody ||= String.new.tap do |rbody|
         @body.each { |part| rbody << part.to_s if part }
         @body.close if @body.respond_to? :close
       end

lib/lamby/ssm_parameter_store.rb

@@ -68,7 +68,7 @@ def dotenv_file
     end
 
     def dotenv_contents
-      params.each_with_object('') do |param, contents|
+      params.each_with_object(String.new) do |param, contents|
         line = "#{param.env}=#{param.value}\n"
         contents << line
       end