migrations: Add the reftable migration to list of migrations

The reftable migration was only being tested out in the dry-run state.
To run the migration live, we need to add the migration to the list of
migration in 'serve.go'. Let's do that.

Note: This change is immutable. Future migrations will only can only
come after (id > 1). This makes the reftable migration rollout a blocker
for all upcoming migrations and as long as it is disabled no other
migration can be applied.