Add GH action step to test shadowJar

[psx95]

Fixes #308

Also adds a GitHub Action to verify that shadowJar builds successfully. This will help prevent regressions during dependency bumps/changes.

Testing

• Ensures that the added GitHub Action fails with the expected message before adding the fix.

    FAILURE: Build failed with an exception.
  
    * What went wrong:
    Execution failed for task ':exporter-auto:shadowJar'.
    > Unsupported class file major version 65
    
    * Try:
    > Run with --stacktrace option to get the stack trace.
    > Run with --info or --debug option to get more log output.
    > Run with --scan to get full insights.
    > Get more help at https://help.gradle.org./
    
    BUILD FAILED in 1m 30s
    Error: Execution failed for task ':exporter-auto:shadowJar'.
    > Unsupported class file major version 65

• GitHub action passes after the fix for failing shadowJar build is implemented.

Note: See issue details for reasoning behind the fix.