Enable backwards compatibility with Java 8 for build-tools

[mark-vieira]

The elasticsearch-hadoop project is still on Java 8 for a number reasons but requires the use of the build-tools project built from the elasticseach source. A previous attempt to make this compatible didn't actually work and we are still publishing JDK 10 target source to artifactory for this project. This PR changes the build-tools project target compatibility to Java 8, and introduces a flag to disable the --release compiler argument so that we can still use the configured compiler Java home bootstrap classpath. This is because we rely on JDK9+ APIs in buildSrc but elasticsearch-hadoop and others don't actually use any of those classes.

[elasticmachine]

Pinging @elastic/es-core-infra

[alpar-t]

We used to have a separate sourceset:

src/main/minimumRuntime
src/main/minimumRuntime/org
src/main/minimumRuntime/org/elasticsearch
src/main/minimumRuntime/org/elasticsearch/gradle
src/main/minimumRuntime/org/elasticsearch/gradle/Version.java
src/main/minimumRuntime/org/elasticsearch/gradle/LoggedExec.java
src/main/minimumRuntime/org/elasticsearch/gradle/VersionProperties.java
src/main/minimumRuntime/org/elasticsearch/gradle/JdkJarHellCheck.java

Seems like this was removed in #41392 even trough it doesn't seem directly related to that change ( does it really have a significant impact on configuration time ?) and we missed in at that time. I think it might be cleaner to add this back and continue to target these few classes at an earlier release. It will also make backporting easier.

[mark-vieira]

I think it might be cleaner to add this back and continue to target these few classes at an earlier release. It will also make backporting easier.

I think you're right. I ditched that because I thought we had fixed this issue by just tweaking the compatibility versions but I see now it was more complex than that. I'll close this PR and open another one to add the minimumRuntime sourceset back.