Remove DX11 from doc for backends

This was removed in WGPU v0.19

crates/kas-wgpu/src/options.rs

@@ -97,7 +97,7 @@ impl Options {
                 "FALLBACK" => Backends::empty(),
                 other => {
                     log::error!("from_env: bad var KAS_BACKENDS={other}");
-                    log::error!("from_env: supported backends: VULKAN, GL, METAL, DX11, DX12, BROWSER_WEBGPU, PRIMARY, SECONDARY, FALLBACK");
+                    log::error!("from_env: supported backends: VULKAN, GL, METAL, DX12, BROWSER_WEBGPU, PRIMARY, SECONDARY, FALLBACK");
                     self.backends
                 }
             }