Although disallowed warnings are likely to be deprecator-specific, it
can be useful to set this value once and have it applied to any
deprecators that are added later.  For example, setting the value in an
initializer via `config.active_support.disallowed_deprecation_warnings`
and applying it to any third-party deprecators that are added later in
the boot process.  Also, when using `:all`, it is more convenient to
write `deprecators.disallowed_warnings = :all` instead of
`deprecators.each { |deprecator| deprecator.disallowed_warnings = :all }`.