/**
     * Escapes special characters in the given XML string to their corresponding
     * XML entities. This method replaces the following characters:
     * <ul>
     *   <li>&amp; with &amp;amp;</li>
     *   <li>&lt; with &amp;lt;</li>
     *   <li>&gt; with &amp;gt;</li>
     *   <li>" with &amp;quot;</li>
     *   <li>' with &amp;apos;</li>
     * </ul>
     * Additionally, it strips invalid XML characters from the input string.