replace maplibregl.supported, that's removed in v3

Use the new example code from https://maplibre.org/maplibre-gl-js/docs/examples/check-for-support/

public/templates/viewer.tmpl

@@ -33,11 +33,31 @@
   <h1 style="display:none;">{{name}}</h1>
   <div id='map'></div>
   <script>
+    function isWebglSupported() {
+        if (window.WebGLRenderingContext) {
+            const canvas = document.createElement('canvas');
+            try {
+                // Note that { failIfMajorPerformanceCaveat: true } can be passed as a second argument
+                // to canvas.getContext(), causing the check to fail if hardware rendering is not available. See
+                // https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/getContext
+                // for more details.
+                const context = canvas.getContext('webgl2') || canvas.getContext('webgl');
+                if (context && typeof context.getParameter == 'function') {
+                    return true;
+                }
+            } catch (e) {
+                // WebGL is supported, but disabled
+            }
+            return false;
+        }
+        // WebGL not supported
+        return false;
+    }
     var q = (location.search || '').substr(1).split('&');
     var preference =
       q.indexOf('vector') >= 0 ? 'vector' :
         (q.indexOf('raster') >= 0 ? 'raster' :
-          (maplibregl.supported() ? 'vector' : 'raster'));
+          (isWebglSupported() ? 'vector' : 'raster'));
 
     var keyMatch = location.search.match(/[\?\&]key=([^&]+)/i);
     var keyParam = keyMatch ? '?key=' + keyMatch[1] : '';