Add aria-label to the version selector component to raise a11y lighthouse score (#1246)

raises score from 96 -> 100 on home page, and 96 -> 100 on /tutorials

Author: AndrewJakubowicz

packages/lit-dev-content/site/_includes/site-nav.html

@@ -2,7 +2,7 @@
    <a href="{{ site.baseurl }}/docs/">Documentation</a>
 
    <litdev-version-selector>
-      <select name="version" autocomplete="off">
+      <select name="version" autocomplete="off" aria-label="Lit version selector">
         {% for version, versionData in versions %}
           <option value="{{ version }}"
             {% if version == selectedVersion %}