[rb] Add base64 gem as a runtime dependency

Ruby 3.3 shows a warning if requiring base64 without specifying it in the gemspec
Ruby 3.4 will throw a warning.
See https://github.com/ruby/ruby/blob/master/doc/NEWS/NEWS-3.3.0.md#stdlib-updates

Closes SeleniumHQ#13447

rb/selenium-webdriver.gemspec

@@ -49,6 +49,7 @@ Gem::Specification.new do |s|
   s.bindir = 'bin'
   s.require_paths = ['lib']
 
+  s.add_runtime_dependency 'base64'
   s.add_runtime_dependency 'rexml', ['~> 3.2', '>= 3.2.5']
   s.add_runtime_dependency 'rubyzip', ['>= 1.2.2', '< 3.0']
   s.add_runtime_dependency 'websocket', ['~> 1.0']