Synology Certificate needs to be RSA instead of ECC based

### What is the issue?

Synology (at least DSM version 6) doesn't support ECC based certificates yet, so the Let's Encrypt certificate needs to be RSA based.

### Steps to reproduce

```
admin:~$ sudo /var/packages/Tailscale/target/bin/tailscale configure synology-cert
Password:
error response from "import" method of "SYNO.Core.Certificate" API: &{5598 }
```

### Are there any recent changes that introduced the issue?

Let's Encrypt recently switched their default to ECDSA instead of RSA:
https://community.letsencrypt.org/t/ecdsa-availability-in-production-environment/150679

### OS

Synology

### OS version

Synology DSM 6

### Tailscale version

1.69.127

### Other software

_No response_

### Bug report

BUG-7553dd3da95eafcff8d57500a0e311a6bf7e074dc8907793af182f4cb82c8131-20240710075237Z-f56f5ad9c9910824