Make examples/instrumentation-quickstart a standalone gradle build

[aabmass]

This PR ports the example from Maven to Gradle. This build is independent from the rest of the repo (not a subproject) to

• minimize the user's mental overhead thinking about the rest of the repo
• use newer java version for Spring Boot 3 (later PR)
• Docker build doesn't need build context at the repo root

[aabmass]

Ignoring failing "ops-java-quickstart-integration-test" job as it is being added in the next PR