Remove github.com/jackc/pgx/v4 dependency

sqlc's own code no longer needs pgx/v4 at build time:

- Delete internal/sqltest/pgx.go — the PostgreSQLPgx helper had no callers.
- Migrate the sqlc-pg-gen internal tool from pgx/v4 to pgx/v5; the
  Connect/Query/Scan API surface used here is identical.

pgx/v4 remains a supported code-generation target for sqlc users
(SQLDriverPGXV4 constant and the imports emitted into generated code
are unchanged); only sqlc's own dependency on the library is removed.
go mod tidy also drops chunkreader/v2, pgconn, pgio, pgproto3/v2,
pgtype, and golang.org/x/crypto as transitively unused.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: kyleconroy

go.mod

@@ -12,7 +12,6 @@ require (
 	github.com/go-sql-driver/mysql v1.9.3
 	github.com/google/cel-go v0.27.0
 	github.com/google/go-cmp v0.7.0
-	github.com/jackc/pgx/v4 v4.18.3
 	github.com/jackc/pgx/v5 v5.9.0
 	github.com/jinzhu/inflection v1.0.0
 	github.com/lib/pq v1.12.0
@@ -36,13 +35,8 @@ require (
 	cel.dev/expr v0.25.1 // indirect
 	filippo.io/edwards25519 v1.1.1 // indirect
 	github.com/inconshreveable/mousetrap v1.1.0 // indirect
-	github.com/jackc/chunkreader/v2 v2.0.1 // indirect
-	github.com/jackc/pgconn v1.14.3 // indirect
-	github.com/jackc/pgio v1.0.0 // indirect
 	github.com/jackc/pgpassfile v1.0.0 // indirect
-	github.com/jackc/pgproto3/v2 v2.3.3 // indirect
 	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
-	github.com/jackc/pgtype v1.14.0 // indirect
 	github.com/jackc/puddle/v2 v2.2.2 // indirect
 	github.com/ncruces/julianday v1.0.0 // indirect
 	github.com/pingcap/errors v0.11.5-0.20240311024730-e056997136bb // indirect
@@ -55,7 +49,6 @@ require (
 	go.uber.org/atomic v1.11.0 // indirect
 	go.uber.org/multierr v1.11.0 // indirect
 	go.uber.org/zap v1.27.0 // indirect
-	golang.org/x/crypto v0.48.0 // indirect
 	golang.org/x/exp v0.0.0-20250620022241-b7579e27df2b // indirect
 	golang.org/x/net v0.49.0 // indirect
 	golang.org/x/sys v0.41.0 // indirect