Update gradle wrapper for gradle v7

[psx95]

Follow up to PR #171

Gradle wrapper was not updated to run with gradle v7. As a result, all tasks run using the wrapper - ./gradlew <taskName> were still being run using gradle v6.

This PR updates gradle wrapper to use gradle v7 as well. Following recommended steps from - Upgrading your build from Gradle 6.x to the latest.

Steps

• Ran gradle help --warning-mode=all
  • Resulted in a single deprecation regarding mainClassName (API would be removed in Gradle 8.0). Addressed in this PR.
• Updated the shadowJar plugin version to be compatible with Gradle 7.
• Ran gradle wrapper --gradle-version 7.0 to update Project to Gradle 7.0.

Testing

• Ran gradle :e2e-test-server:build --warning-mode all → successful and no deprecation warnings.
• Ran ./gradlew shadowJar → successful and generated a fatJar through which main class was invoked using -
  • java -cp ./e2e-test-server/build/libs/e2e-test-server-0.1.0-SNAPSHOT-all.jar com.google.cloud.opentelemetry.endtoend.Server

Fixes #108