MDL-69951 filter_tidy: Replace drop-font-tags

The option drop-font-tags was removed in HTML Tidy version 5.5.14.  It
had been deprecated since 5.1.25, the docs said "Use the clean option
instead" which is what this commit does (see
https://api.html-tidy.org/tidy/quickref_5.1.25.html#drop-font-tags).

filter/tidy/filter.php

@@ -47,7 +47,7 @@ function filter($text, array $options = array()) {
                  'show-body-only' => true,
                  'tidy-mark' => false,
                  'drop-proprietary-attributes' => true,
-                 'drop-font-tags' => true,
+                 'clean' => true,
                  'drop-empty-paras' => true,
                  'indent' => true,
                  'quiet' => true,