Change Default CI version to JDK 11 (opensearch-project#290)

* change default CI version to JDK 11, add JDK versions to CI matrix and document changes in developer guide

Signed-off-by: David Cui <davidcui@amazon.com>

.github/workflows/reports-scheduler-test-and-build-workflow.yml

@@ -7,13 +7,18 @@ env:
 
 jobs:
   build:
+    strategy:
+      matrix:
+        java:
+          - 11
+          - 14
     runs-on: ubuntu-latest
 
     steps:
-      - name: Set up JDK 1.14
+      - name: Set up JDK ${{ matrix.java }}
         uses: actions/setup-java@v1
         with:
-          java-version: 1.14
+          java-version: ${{ matrix.java }}
 
       # reports-scheduler
       - name: Checkout Reports Scheduler

DEVELOPER_GUIDE.md

@@ -2,6 +2,19 @@
 
 So you want to contribute code to this project? Excellent! We're glad you're here. Here's what you need to do.
 
+## Install Prerequisites
+
+### JDK 11
+
+OpenSearch builds using Java 11 at a minimum. This means you must have a JDK 11
+installed with the environment variable `JAVA_HOME` referencing the path to Java home
+for your JDK 11 installation, e.g. `JAVA_HOME=/usr/lib/jvm/jdk-11`.
+
+By default, tests use the same runtime as `JAVA_HOME`. However, since OpenSearch
+supports JDK 8, the build supports compiling with JDK 11 and testing on a different
+version of JDK runtime. To do this, set `RUNTIME_JAVA_HOME` pointing to the Java home of
+another JDK installation, e.g. `RUNTIME_JAVA_HOME=/usr/lib/jvm/jdk-8`.
+
 ## Setup
 
 1. Download OpenSearch for the version that matches the [OpenSearch Dashboards version specified in package.json](./package.json#L7).