[Encore] Updating Babel Plugin

Author: ThomasLandauer

frontend/encore/installation.rst

@@ -111,7 +111,7 @@ is the main config file for both Webpack and Webpack Encore:
         .enableVersioning(Encore.isProduction())
 
         .configureBabel((config) => {
-            config.plugins.push('@babel/plugin-proposal-class-properties');
+            config.plugins.push('@babel/plugin-transform-class-properties');
         })
 
         // enables @babel/preset-env polyfills