Blockage migration from level 4 yields invalid credential

If the user's Lox credential has trust_level 3, blockage migration succeeds. If the credential has trust_level 4, the user ends up with a new credential, but the LA can't verify this new credential.

Demo: vecna/lox@59358787 on branch blockage_migration_test

cargo test blockage_migration_level_3 passes, cargo test blockage_migration_level_4 fails.

Migrating from a level 4 credential works in https://git-crysp.uwaterloo.ca/iang/lox, so this issue must have been introduced by some change since then.