Document the defaults of cache_format_version

And add its new default to the new_framework_defaults_7_0 file.

guides/source/configuring.md

@@ -867,7 +867,7 @@ There are a few configuration options available in Active Support:
 
 * `config.active_support.use_authenticated_message_encryption` specifies whether to use AES-256-GCM authenticated encryption as the default cipher for encrypting messages instead of AES-256-CBC.
 
-* `config.active_support.cache_format_version` specifies which version of the cache serializer to use. Possible values are `6.1` and `7.0`. Defaults to `6.1`.
+* `config.active_support.cache_format_version` specifies which version of the cache serializer to use. Possible values are `6.1` and `7.0`.
 
 * `ActiveSupport::Logger.silencer` is set to `false` to disable the ability to silence logging in a block. The default is `true`.
 
@@ -1061,6 +1061,7 @@ text/javascript image/svg+xml application/postscript application/x-shockwave-fla
 - `config.action_view.apply_stylesheet_media_default` : `false`
 - `config.active_support.key_generator_hash_digest_class`: `OpenSSL::Digest::SHA256`
 - `config.active_support.hash_digest_class`: `OpenSSL::Digest::SHA256`
+- `config.active_support.cache_format_version`: `7.0`
 - `config.action_dispatch.return_only_request_media_type_on_content_type`: `false`
 
 #### For '6.1', defaults from previous versions below and:
@@ -1137,6 +1138,7 @@ text/javascript image/svg+xml application/postscript application/x-shockwave-fla
 - `config.active_support.use_authenticated_message_encryption`: `false`
 - `config.active_support.hash_digest_class`: `OpenSSL::Digest::MD5`
 - `config.active_support.key_generator_hash_digest_class`: `OpenSSL::Digest::SHA1`
+- `config.active_support.cache_format_version`: `6.1`
 - `config.action_dispatch.return_only_request_media_type_on_content_type`: `true`
 - `ActiveSupport.utc_to_local_returns_utc_offset_times`: `false`
 

railties/lib/rails/generators/rails/app/templates/config/initializers/new_framework_defaults_7_0.rb.tt

@@ -27,3 +27,10 @@
 # various cache keys leading to cache invalidation.
 #
 # Rails.application.config.active_support.hash_digest_class = OpenSSL::Digest::SHA256
+
+# Change the format of the cache entry.
+# Changing this default means that all new cache entries added to the cache
+# will have a different format that is not support by Rails 6.1 applications.
+# Only change this value after your application is fully deployed to Rails 7.0
+# and you have no plans to rollback.
+# config.active_support.cache_format_version = 7.0