Mark PHP 8.4 as latest release in build config generator

Author: alcaeus

.evergreen/config/generate-config.php

@@ -21,9 +21,7 @@
     '4.2',
 ];
 
-// TODO: Change when PHP 8.4 is stable
-// $latestPhpVersion = max($phpVersions);
-$latestPhpVersion = '8.3';
+$latestPhpVersion = max($phpVersions);
 
 // Only test the latest PHP version for libmongoc
 $libmongocBuildPhpVersions = [ $latestPhpVersion ];