Fix jvm version compatibility

[Tapchicoma]

Interesting that presence of Kover plugin hides the resolution error and Gradle just silently continue the build. Check this build scan - dependencies tab is empty. I highly suspect due to these additional Kover attributes.

If I remove Kover from this project - I see proper Gradle failure:

Could not determine the dependencies of task ':kotlinx-serialization-protobuf:compileTestKotlinJvm'.
> Could not resolve all task dependencies for configuration ':kotlinx-serialization-protobuf:jvmTestCompileClasspath'.
   > Could not resolve project :kotlinx-serialization-protobuf:proto-test-model.
     Required by:
         project :kotlinx-serialization-protobuf
      > No matching variant of project :kotlinx-serialization-protobuf:proto-test-model was found. The consumer was configured to find a library for use during compile-time, compatible with Java 8, preferably in the form of class files, preferably optimized for standard JVMs, and its dependencies declared externally, as well as attribute 'org.jetbrains.kotlin.platform.type' with value 'jvm' but:
          - Variant 'apiElements' declares a library for use during compile-time, packaged as a jar, and its dependencies declared externally:
              - Incompatible because this component declares a component, compatible with Java 11 and the consumer needed a component, compatible with Java 8
              - Other compatible attributes:
                  - Doesn't say anything about its target Java environment (preferred optimized for standard JVMs)
                  - Doesn't say anything about org.jetbrains.kotlin.platform.type (required 'jvm')
          - Variant 'mainSourceElements' declares a component, and its dependencies declared externally:
              - Incompatible because this component declares a component of category 'verification' and the consumer needed a library
              - Other compatible attributes:
                  - Doesn't say anything about its elements (required them preferably in the form of class files)
                  - Doesn't say anything about its target Java environment (preferred optimized for standard JVMs)
                  - Doesn't say anything about its target Java version (required compatibility with Java 8)
                  - Doesn't say anything about its usage (required compile-time)
                  - Doesn't say anything about org.jetbrains.kotlin.platform.type (required 'jvm')
          - Variant 'runtimeElements' declares a library for use during runtime, packaged as a jar, and its dependencies declared externally:
              - Incompatible because this component declares a component, compatible with Java 11 and the consumer needed a component, compatible with Java 8
              - Other compatible attributes:
                  - Doesn't say anything about its target Java environment (preferred optimized for standard JVMs)
                  - Doesn't say anything about org.jetbrains.kotlin.platform.type (required 'jvm')
          - Variant 'testResultsElementsForTest':
              - Incompatible because this component declares a component of category 'verification' and the consumer needed a library
              - Other compatible attributes:
                  - Doesn't say anything about how its dependencies are found (required its dependencies declared externally)
                  - Doesn't say anything about its elements (required them preferably in the form of class files)
                  - Doesn't say anything about its target Java environment (preferred optimized for standard JVMs)
                  - Doesn't say anything about its target Java version (required compatibility with Java 8)
                  - Doesn't say anything about its usage (required compile-time)
                  - Doesn't say anything about org.jetbrains.kotlin.platform.type (required 'jvm')

[shanshin]

I highly suspect due to these additional Kover attributes.

That's not quite true.
It's all about the specifics of the dependency resolution in Gradle. If there is no dependency with the requested attributes, then an arbitrary dependency with disjoint attributes is taken.

This is the expected behavior of Gradle, see gradle/gradle#27019.

In recent versions of Kover, an attribute was added that eliminated this "hiding of problems".
See relative issue in Kover: Kotlin/kotlinx-kover#728

[Tapchicoma]

Should Kover version in this project be updated to at least 0.9.0?