Downscale test_js_prev_lts to large (#38855)

Summary:
Pull Request resolved: #38855

For the `test_js_prev_lts` job we don't need xlarge resources.

The current average usage for `test_js_prev_lts` is around 25%.

We can also downscale to medium in few days if this scales well.

**Insights Dashboard:**

`test_js_prev_lts`

{F1065747013}

Changelog:
[Internal] [Changed] - Downscale test_js_prev_lts job to Large

Reviewed By: cortinico, cipolleschi

Differential Revision: D48125886

fbshipit-source-id: 521fac38a1d699e576bc1ce5a3a29a47bc284e76

.circleci/config.yml

@@ -142,7 +142,7 @@ executors:
     <<: *defaults
     docker:
       - image: *nodeprevlts_image
-    resource_class: "xlarge"
+    resource_class: "large"
   # Executor with Node & Java used to inspect and lint
   node-browsers-small:
     <<: *defaults