Reserve 5 migrations for backports

Reserve 5 migrations so that we have the option of backporting fixes
that include a db migration to the stable/icehouse branch.

The choice of 5 migrations is just a magic number. We've never provide
backports of migrations, so 5 seems to be enough.

Change-Id: Iedb03bb4fd6feb5e5061b24fb30f710e8e83d350
Merge "Reserve 5 migrations for backports"