break:remove the db-use-legacy-gucs config

BREAKING CHANGE

All PostgreSQL versions will use JSON GUCs for headers, cookies and JWT claims.

test/io/configs/expected/no-defaults-with-db.config

@@ -17,7 +17,6 @@ db-config = true
 db-pre-config = "postgrest.preconf"
 db-tx-end = "commit-allow-override"
 db-uri = "postgresql://"
-db-use-legacy-gucs = false
 jwt-aud = "https://example.org"
 jwt-role-claim-key = ".\"a\".\"role\""
 jwt-secret = "OVERRIDE=REALLY=REALLY=REALLY=REALLY=VERY=SAFE"