Add AmazonLinux-2023 as alias for RHEL-9

AmazonLinux-2023 is compatible with RHEL/CentOS/Rocky/etc v9 but currently we get

```sh
rig add 4.5.2
# [ERROR] Failed to resolve R version: Cannot download JSON: HTTP status client error (404 Not Found) for url (https://api.r-hub.io/rversions/resolve/4.5.2/linux-amzn-2023/aarch64)
```