Scanning multi-dimensional array into `any` or `[]any` results in a flattened slice

**Describe the bug**
When scanning a multi-dimensional postgres array into `any` or `[]any` the array will be flattened.

**To Reproduce**
Steps to reproduce the behavior:

- SELECT a multi-dimensional array
- Scan it into `any` or `[]any`
- Print

See example below:

```go
package main

import (
"context"
"log"
"os"
"fmt"

"github.com/jackc/pgx/v5"
)

func main() {
conn, err := pgx.Connect(context.Background(), os.Getenv("DATABASE_URL"))
if err != nil {
log.Fatal(err)
}
defer conn.Close(context.Background())

var foo any
var pop []any
var zing [][]any

row := conn.QueryRow(context.Background(), "SELECT '{{1,11},{2,22}}'::bigint[], '{{3,33},{4,44}}'::bigint[], '{{5,55},{6,66}}'::bigint[]")
err := row.Scan(&foo, &pop, &zing)
if err != nil {
t.Error(err)
}

fmt.Printf("foo: %+v", foo) // The nested array is flattened (type = interface{} | []interface{} | interface{} | int64)
fmt.Printf("pop: %+v", pop) // The nested array is flattened (type = []interface{} | interface{} | int64)
fmt.Printf("zing: %+v", zing) // Only zing has the desired form (type = [][]interface{} | []interface{} | interface{} | int64)

// PRINTS:
// foo: [1 11 2 22] <- FLATTENED, NOT OK
// pop: [3 33 4 44] <- FLATTENED, NOT OK
// zing: [[5 55] [6 66]] <- GOOD
}
```

**Expected behavior**
Regardless of destination type (any, []any or [][]any) the array should NOT be flattened

**Actual behavior**
When scanning into any or []any the array is flattened

**Version**
- Go: 1.22
- PostgreSQL: PostgreSQL 16.1 on x86_64-pc-linux-musl, compiled by gcc (Alpine 13.2.1_git20231014) 13.2.1 20231014, 64-bit
- pgx: v5.5.5

No repository file or test is named. Start by running the provided Go reproduction against PostgreSQL and compare scanning into any, []any, and [][]any. Done means multi-dimensional arrays retain their nested shape for all three destination types, with coverage for the reported behavior.

Written by the indexing model from the issue text.