[api-minor] Rename -es5 to -legacy, to reduce confusion over what's actually supported (issue 12976)

*Please note that this will also require some edits of the Wiki.*

src/core/worker.js

@@ -147,7 +147,7 @@ class WorkerMessageHandler {
         throw new Error(
           "The browser/environment lacks native support for critical " +
             "functionality used by the PDF.js library (e.g. `ReadableStream`); " +
-            "please use an `es5`-build instead."
+            "please use a `legacy`-build instead."
         );
       }
     }