Update dep versions to work with latest build.xml (#2702)

Previously, building the demo (ant -f javascript/build.xml compile-demo) would fail because the latest build.xml is incompatible with the version of closure-compiler we recommend here. With this change, the build will work again.

Co-authored-by: Tijana Vislavski Gradina <tijanavg@google.com>

javascript/README.md

@@ -20,9 +20,9 @@ How to setup:
 
 1.  We officially support only these versions of these dependencies:
 
-* Closure library: v20190415
+* Closure library: v20201006
 
-* Closure compiler: v20140407
+* Closure compiler: v20210302
 
 * Closure linter: v2.3.19