Remove the unusable dc-test-server Flyway target/environment

The dc-test-server target in the economy-flyway migrate/info workflows will never work, so remove it: deleted the dc-test-server GitHub Environment (and its blank FLYWAY_* secrets), and dropped the dc-test-server choice option from economy-flyway-migrate.yml and economy-flyway-info.yml (and the env list comment). Remaining targets: democracycraft, statecraft (+ the staging-clean environment for the clean workflow).