Remove deprecated base64 in favour of pack/unpack

The base64 library is external as of Ruby 3.4 and is no longer available
by default in Ruby. The pack/unpack methods support encoding as base64
and can be used in it's place.

String#unpack.first was preferred here to support Ruby 2.3, which didn't
yet have String#unpack1 introduced in Ruby 2.4.

lib/rack/auth/basic.rb

@@ -2,7 +2,6 @@
 
 require_relative 'abstract/handler'
 require_relative 'abstract/request'
-require 'base64'
 
 module Rack
   module Auth
@@ -48,7 +47,7 @@ def basic?
         end
 
         def credentials
-          @credentials ||= Base64.decode64(params).split(':', 2)
+          @credentials ||= params.unpack("m").first.split(':', 2)
         end
 
         def username

lib/rack/auth/digest/nonce.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 require 'digest/md5'
-require 'base64'
 
 module Rack
   module Auth
@@ -21,15 +20,15 @@ class << self
         end
 
         def self.parse(string)
-          new(*Base64.decode64(string).split(' ', 2))
+          new(*string.unpack("m").first.split(' ', 2))
         end
 
         def initialize(timestamp = Time.now, given_digest = nil)
           @timestamp, @given_digest = timestamp.to_i, given_digest
         end
 
         def to_s
-          Base64.encode64("#{@timestamp} #{digest}").strip
+          ["#{@timestamp} #{digest}"].pack("m0")
         end
 
         def digest

lib/rack/session/cookie.rb

@@ -4,7 +4,6 @@
 require 'zlib'
 require_relative 'abstract/id'
 require 'json'
-require 'base64'
 require 'delegate'
 
 module Rack
@@ -51,11 +50,11 @@ class Cookie < Abstract::PersistedSecure
       # Encode session cookies as Base64
       class Base64
         def encode(str)
-          ::Base64.strict_encode64(str)
+          [str].pack("m0")
         end
 
         def decode(str)
-          ::Base64.decode64(str)
+          str.unpack("m").first
         end
 
         # Encode session cookies as Marshaled Base64 data