This commit adds support for `:message_pack` as an option for
`config.active_support.cache_format_version`.

Cache entries written using the `6.1` or `7.0` formats can be read when
using the `:message_pack` format. Additionally, cache entries written
using the `:message_pack` format can now be read when using the `6.1` or
`7.0` format. These behaviors makes it easy to migrate between formats
without invalidating the entire cache.