Silence msgpack warnings when detecting format

This silences a "ActiveSupport::MessagePack requires the msgpack gem"
warning that can occur when using a non-`:message_pack` serializer and
trying to detect the format of a serialized cache entry.

`silence_warnings` is a blunt instrument, but this `require` is only for
decoding legacy cache entries that were encoded using `:message_pack`,
if any.  (i.e. If `:message_pack` is currently being used as a
serializer, then `SerializerWithFallback::[]` should have already loaded
`active_support/message_pack`.)  Therefore, it seems less hazardous to
inadvertently silence other warnings that may occur when loading the
file.

Co-authored-by: Alex Ghiculescu <alex@tanda.co>

activesupport/lib/active_support/cache/serializer_with_fallback.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "active_support/core_ext/kernel/reporting"
+
 module ActiveSupport
   module Cache
     module SerializerWithFallback # :nodoc:
@@ -156,7 +158,7 @@ def dumped?(dumped)
           private
             def available?
               return @available if defined?(@available)
-              require "active_support/message_pack"
+              silence_warnings { require "active_support/message_pack" }
               @available = true
             rescue LoadError
               @available = false