Make resource-detector compatible with Java 8

[psx95]

Description

This PR makes the resource detector Java 8 compatible. This is to support #266.

How the changes were tested

• Used the GCP Detector jar built from this change in a Java 8 application (running in a docker container) to successfully detect resources. The Java 8 application was similar to resource-example in the current repo.
• Used the javap tool to check the major version with which the classes was compiled -

  javap -v javap -v detectors/resources/build/classes/java/main/com/google/cloud/opentelemetry/detectors/*.class | grep major

  Output -

  major version: 52
  major version: 52
  major version: 52
  major version: 52

Indicates all classes within resource detector are Java 8 compatible.

Other artifacts are still compiled & targeted for Java 11.

javap -v exporters/*/build/classes/java/main/com/google/cloud/opentelemetry/*/*.class | grep major

Output -

  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55
  major version: 55

[dashpole]

Do we have any test/checks to ensure it continues to support java 8? Or is this enough, as long as we don't change this config?

[psx95]

Do we have any test/checks to ensure it continues to support java 8? Or is this enough, as long as we don't change this config?

No, not at the moment, I tested everything manually in a one-off fashion. We currently do not have any tests pertaining to specific Java version.

This was just the first step, checking to see if our build works properly. Once we refactor out the resource-detector-support library, we can add tests there.

However, based on the gradle documentation on release flag used in this config, I think it should give us better assurances than using the Java compatibility options.

Maybe @jsuereth can weigh in here