Previously we used allow bucket policies without
`Version` field to be set to any given value, but
this behavior is inconsistent with AWS S3.
PR #5790 addressed this by making bucket policies
stricter and cleaner, but this causes a breaking
change causing any existing policies perhaps without
`Version` field or the field to be empty to fail upon
server startup.
This PR brings a code to migrate under these scenarios
as a one time operation.