Fix gradle dependency error for publishing to maven local

[hpmellema]

Background

• Corrects an issue introduced in PR: Use new gradle plugins for validation model and protocol tests #2176

• Gradle requires an explicit dependency on smithyJarStaging for the publish task. Before this commit the following error was encountered when running ./gradlew publishToMavenLocal:

* What went wrong:
A problem was found with the configuration of task ':smithy-aws-protocol-tests:sourcesJar' (type 'Jar').
  - Gradle detected a problem with the following location: '/Users/example/code/smithy/smithy-aws-protocol-tests/build/tmp/staging-smithyJarStaging'.
    
    Reason: Task ':smithy-aws-protocol-tests:sourcesJar' uses this output of task ':smithy-aws-protocol-tests:smithyJarStaging' without declaring an explicit or implicit dependency. This can lead to incorrect results being produced, depending on what order the tasks are executed.
    
    Possible solutions:
      1. Declare task ':smithy-aws-protocol-tests:smithyJarStaging' as an input of ':smithy-aws-protocol-tests:sourcesJar'.
      2. Declare an explicit dependency on ':smithy-aws-protocol-tests:smithyJarStaging' from ':smithy-aws-protocol-tests:sourcesJar' using Task#dependsOn.
      3. Declare an explicit dependency on ':smithy-aws-protocol-tests:smithyJarStaging' from ':smithy-aws-protocol-tests:sourcesJar' using Task#mustRunAfter.

Testing

• Successfully ran ./gradlew publishToMavenLocal

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