Add version field to the XML reporter output

We start at version 2, with version 1 being taken up by the output
from Catch2 v2.

src/catch2/reporters/catch_reporter_xml.cpp

@@ -56,6 +56,7 @@ namespace Catch {
         m_xml.startElement("Catch2TestRun")
              .writeAttribute("name"_sr, m_config->name())
              .writeAttribute("rng-seed"_sr, m_config->rngSeed())
+             .writeAttribute("xml-format-version"_sr, 2)
              .writeAttribute("catch2-version"_sr, libraryVersion());
         if ( m_config->testSpec().hasFilters() ) {
             m_xml.writeAttribute( "filters"_sr, m_config->testSpec() );