Auto-generate stable fixture UUIDs on PostgreSQL.
Fixes: #11524
Optionally support the `uuidtools` gem for faster UUID fixture generation.
Add uuid_v3 and uuid_v5 methods to SecureRandom to remove uuidtools dependency.
Clean up primary_key_type API.
Add CHANGELOG entry for `SecureRandom::uuid_v3` and `SecureRandom::uuid_v5`.
Improve documentation legibility per robin850's suggestions.