Add Java8 support

[psx95]

Description

This PR makes the minimum supported version for artifacts generated from this repository to Java 8 by default. Operational modules like e2e-testing, may use higher Java versions due to dependency constraints.

Testing

• ./gradlew clean build successfully runs
• Looking at the class files present in generated module JARs using javap -v path/to/classfile/in/jar reveals major version as 52 (Java 8) and 55 (Java 11) for expected modules.
• Using APIs not available in Java 8 for modules configured to produce Java 8 bytecode result in build error. Note that since test code is not part of the final JAR, tests may still use APIs upto Java 11.

[psx95]

@punya any concerns about not actually testing against Java8? @psx95 mentioned that we weren't doing it before either so this seems reasonable. But I'm no Java expert and don't know what might go wrong

I think the ideal way going forward would be to write a CI that loads the modules/JAR intended to be distributed as Java8 compatible on a VM that only has Java 8 installed on it.

I don't think we should force the current unit tests to use Java8 API.

What might go wrong?

A bump in gradle version or some gradle config changes could cause the --release flag to be ignored, resulting in artifacts that are not Java 8 compatible.

Edit: This would not have been a concern if were using two different JVM toolchains (Java 8 for all projects and Java 11 for e2e-tests). I had previously considered this way, but changed the implementation due to the following reasons:

• Building this project would require the presence of two JDKs on the user's machine.
• Using multiple toolchains seems inefficient and would increase the CloudBuild time since we would need to download and install two versions of Java in all docker images.
• Additionally, docker images would now have to be custom, since I could not find an official docker image for gradle + Java 8 + Java 11.

[psx95]

We're lucky that we hardly took any dependencies on new syntax or libraries in all this time!

Yes! Most of the new syntax is used only in unit tests - https://github.com/GoogleCloudPlatform/opentelemetry-operations-java/pull/353/commits/cfa27e5bc36a543a54e1edf18ebf30ca332db145.