minor #19037 [Encore] Updating Babel Plugin (ThomasLandauer)

This PR was merged into the 6.3 branch.

Discussion
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[Encore] Updating Babel Plugin

I don't know what I'm doing here!
I merely guessed this, with the help of facebook/create-react-app#13189 after I was getting:
> Error: Cannot find package '`@babel`/plugin-proposal-class-properties' imported from ...

Commits
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6ae73f5 [Encore] Updating Babel Plugin

Author: xabbuh

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