Add JDK 15/16 To The CircleCI Build

Though JDK16 is not yet released, building with it now will give us warnings about issues such as failure to build on JDK15 (scala#450)

Author: isomarcte

.circleci/config.yml

@@ -9,10 +9,16 @@ executors:
       - image: circleci/openjdk:11-jdk
   scala_jdk13_executor:
     docker:
-      - image: circleci/openjdk:13.0.1-jdk-buster
+      - image: circleci/openjdk:13-buster
   scala_jdk14_executor:
     docker:
-      - image: circleci/openjdk:14-ea-26-jdk-buster
+      - image: circleci/openjdk:14-buster
+  scala_jdk15_executor:
+    docker:
+      - image: circleci/openjdk:15-buster
+  scala_jdk16_executor:
+    docker:
+      - image: circleci/openjdk:16-buster
 
 commands:
   sbt_cmd:
@@ -128,6 +134,30 @@ workflows:
           name: jdk14_dotty
           java_version: jdk14
           scala_version: 0.27.0-RC1
+      - scala_job:
+          name: jdk15_2.12
+          java_version: jdk15
+          scala_version: 2.12.12
+      - scala_job:
+          name: jdk15_2.13
+          java_version: jdk15
+          scala_version: 2.13.3
+      - scala_job:
+          name: jdk15_dotty
+          java_version: jdk15
+          scala_version: 0.27.0-RC1
+      - scala_job:
+          name: jdk16_2.12
+          java_version: jdk16
+          scala_version: 2.12.12
+      - scala_job:
+          name: jdk16_2.13
+          java_version: jdk16
+          scala_version: 2.13.3
+      - scala_job:
+          name: jdk16_dotty
+          java_version: jdk16
+          scala_version: 0.27.0-RC1
       - scalajs_job:
           name: sjs1.0_2.12
           scala_version: 2.12.12