Bump Node LTS to 14, and Node PrevLTS to 12. (facebook#30237)

Summary:
On October 27, 2020, Node 14 will enter active LTS status, and Node 12 will enter maintenance LTS status at the end of November.

With these upcoming dates in mind, I am bumping the Circle CI NodeLTS executor to use Node 14, and the NodePrevLTS executor to use Node 12.

With these changes, Node 12 will be hereto considered the minimum supported version.

## Changelog

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[Changed] [JavaScript] - Bump minimum Node version to 12 LTS

Pull Request resolved: facebook#30237

Test Plan: Circle CI

Reviewed By: PeteTheHeat

Differential Revision: D24513133

Pulled By: hramos

fbshipit-source-id: 2651d954d546e5f7c9e40b54ef3ee976ad9b2d5e

.circleci/config.yml

@@ -28,11 +28,11 @@ executors:
   nodelts:
     <<: *defaults
     docker:
-      - image: circleci/node:12
+      - image: circleci/node:14
   nodeprevlts:
     <<: *defaults
     docker:
-      - image: circleci/node:10
+      - image: circleci/node:12
   reactnativeandroid:
     <<: *defaults
     docker: