Update Prerequisites in howto docs

As of Clang 19, libc++ doesn't support std::chrono::clock_cast which is
needed to generate the HTML lists if uncommitted XML files for new
issues are present.

how-to-docs.html

@@ -25,7 +25,7 @@ <h1>How To Instructions</h1>
 <h2>Prerequisites</h2>
 <ul>
   <li>Git</li>
-  <li>A C++17 compiler. Clang 9 works. GCC 8.x works for unix-like systems, but GCC 9.3 is recommended (because its <code>std::filesystem</code> supports Windows, and no additional <code>-lstdc++fs</code> linker option is needed).</li>
+  <li>A C++20 compiler. GCC 13 works. Clang 16 mostly works.</li>
   <li>Windows is a prerequisite to use the Windows <code>.bat</code> scripts;
   a POSIX environment is a prerequisite to use the equivalent <code>make</code>
   commands.</li>