Merge release branch into master. (#7517)

* Added background information about proto3 presence. (#7501)

* Fixed bug in map key sorting for Java TextFormat. (#7508)

Fixes: protocolbuffers/protobuf#7505

* Update protobuf version

* Added a changelog entry about the Java fix. (#7516)

package.json

@@ -1,6 +1,6 @@
 {
   "name": "google-protobuf",
-  "version": "3.12.0-rc.2",
+  "version": "3.12.0",
   "description": "Protocol Buffers for JavaScript",
   "main": "google-protobuf.js",
   "files": [