Merge branch '6254-installer-symlink-warnings-mishandling' into 'develop'

Resolve "installer symlink warnings / mishandling(?)"

Closes #6254

See merge request ispconfig/ispconfig3!1541

Author: Marius Burkard

install/lib/installer_base.lib.php

@@ -3133,6 +3133,11 @@ public function make_ispconfig_ssl_cert() {
 						// certbot returns with 0 on issue for already existing certificate
 
 						$acme_cert_dir = '/etc/letsencrypt/live/' . $hostname;
+						foreach (array( $ssl_crt_file, $ssl_key_file) as $f) {
+							if (file_exists($f) && ! is_link($f)) {
+								unlink($f);
+							}
+						}
 						symlink($acme_cert_dir . '/fullchain.pem', $ssl_crt_file);
 						symlink($acme_cert_dir . '/privkey.pem', $ssl_key_file);