Update docs about needing java 11 to build (#412)

CONTRIBUTING.md

@@ -3,10 +3,9 @@
 Pull requests for bug fixes are welcome, but before submitting new features or changes to current functionality [open an issue](https://github.com/open-telemetry/opentelemetry-auto-instr-java/issues/new)
 and discuss your ideas or propose the changes you wish to make. After a resolution is reached a PR can be submitted for review.
 
-In order to fully build and test this whole repository you need the following:
-* Installed both JDK 8 and 9.
-* Java 8 should be set as default: `java -version` should give you version 8.
-* Defined environment variables `JAVA_8_HOME` and `JAVA_9_HOME` which point to the corresponding java homes. 
+In order to build and test this whole repository you need JDK 11+.
+Some instrumentations and tests may put constraints on which java versions they support.
+See [Executing tests with specific java version](#Executing tests with specific java version) below.
 
 ### Plugin structure
 OpenTelemetry Auto Instrumentation java agent's jar can logically be divided into 3 parts.

instrumentation/akka-http-10.0/akka-http-10.0.gradle

@@ -112,6 +112,7 @@ compileLatestDepTestGroovy {
 // They all use the same constraints as the main test source set.
 lagomTest {
   onlyIf {
-    executable == toExecutable(findJavaHome(JavaVersion.VERSION_1_8))
+    String java8Home = findJavaHome(JavaVersion.VERSION_1_8)
+    java8Home != null && executable == toExecutable(java8Home)
   }
 }

instrumentation/java-concurrent/java-concurrent.gradle

@@ -40,6 +40,7 @@ test.dependsOn slickTest
 // force this one test to run on java8 only
 slickTest {
   onlyIf {
-    executable == toExecutable(findJavaHome(JavaVersion.VERSION_1_8))
+    String java8Home = findJavaHome(JavaVersion.VERSION_1_8)
+    java8Home != null && executable == toExecutable(java8Home)
   }
 }

instrumentation/jaxrs/jaxrs-2.0/jaxrs-2.0.gradle

@@ -56,6 +56,7 @@ test.dependsOn resteasy31Test
 // force this one test to run on java8 only
 resteasy31Test {
   onlyIf {
-    executable == toExecutable(findJavaHome(JavaVersion.VERSION_1_8))
+    String java8Home = findJavaHome(JavaVersion.VERSION_1_8)
+    java8Home != null && executable == toExecutable(java8Home)
   }
 }