Port the code to PHP 8.5.

Replace the short open tags and the removed functions (mcrypt, mysql_error,
salt-less crypt). Decode the reCAPTCHA reply as JSON, turn off mysqli
exceptions, drop stripslashes(), and default missing input values.
Fix the misplaced continue in prune_images().

feed.php

@@ -1,4 +1,4 @@
-<?
+<?php
 require_once('./open_db.lib.php');
 open_db('open');
 require_once('./get_data.lib.php');
@@ -13,7 +13,7 @@ header("Content-Type: application/rss+xml");
 
 // Set up a RSS "header"
 $query = "SELECT MAX(`date`) FROM $table;";
-$result = $db->query($query) or die("Query failed : ".$db->error());
+$result = $db->query($query) or die("Query failed : ".$db->error);
 $most_recent = $result->fetch_row();
 $now = date("D, d M Y H:i:s O");