fix: SystemStubs test dependency

[maximearmstrong]

Summary:

This PR:

• removes System Lambda from build.gradle
• updates the implementation of SystemStubs to be a testImplementation
• updates the version of SystemStubs

Expected behavior:

Same as before, but the dependency to SystemStubs won't be packaged in the JAR file.

Please make sure these boxes are checked before submitting your pull request - thanks!

• Run the unit tests with gradle test to make sure you didn't break anything
• Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification(https://www.conventionalcommits.org/en/v1.0.0/).
• Linked all relevant issues
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[barbeau]

Thanks @maximearmstrong!

[barbeau]

Hmmm...some checks are failing - maybe try re-running them?

[maximearmstrong]

Hmmm...some checks are failing - maybe try re-running them?

Yes, for some reason the checks sometimes fail to find com.github.sherter.google-java-format, but succeed when re-run.