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.