Fix issue for frozen strings

lib/capybara/playwright/node.rb

@@ -602,7 +602,7 @@ class PressKey
           def initialize(key:, modifiers:)
             # Shift always requires uppercase key
             # See https://playwright.dev/docs/input#keys-and-shortcuts
-            key.upcase! if modifiers.include?(MODIFIERS[:shift])
+            key = key.upcase if modifiers.include?(MODIFIERS[:shift])
 
             # puts "PressKey: key=#{key} modifiers: #{modifiers}"
             if modifiers.empty?