Remove dependency on base64

Ruby 3.3 shows a warning when using base64 without declaring it in the gemspec.
Ruby 3.4 will remove base64 as a default gem.

lib/webmock/util/headers.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'base64'
-
 module WebMock
 
   module Util
@@ -59,7 +57,8 @@ def self.decode_userinfo_from_header(header)
       end
 
       def self.basic_auth_header(*credentials)
-        "Basic #{Base64.strict_encode64(credentials.join(':')).chomp}"
+        strict_base64_encoded = [credentials.join(':')].pack("m0")
+        "Basic #{strict_base64_encoded.chomp}"
       end
 
       def self.normalize_name(name)

spec/acceptance/async_http_client/async_http_client_spec_helper.rb

@@ -11,7 +11,7 @@ def http_request(method, url, options = {}, &block)
       end
     end
     headers.push(
-      ['authorization', 'Basic ' + Base64.strict_encode64(options[:basic_auth].join(':'))]
+      ['authorization', WebMock::Util::Headers.basic_auth_header(options[:basic_auth])]
     ) if options[:basic_auth]
 
     body = options[:body]