migrator: Reduce the backoff limit to 6hrs

The reftable migrator is an opportunistic migrator which attempts to migrate repositories whenever there are incoming read requests. The migrator cancels any ongoing migrations when a corresponding write request is received.

The max delay for this has been set to 32hrs, which is very conservative. Especially since one Gitaly node can only process one migration request at a time. So let's reduce this to 6hrs.

Add a missing log field while we're here.