[#2391] Add @since tag to Project.findProperty()

[szpak]

Context

In relation to #2391. The method in Project which I was struggling recently in which version was added to declare compatibility range of my plugin.

Related release notes.

Contributor Checklist

• Review Contribution Guidelines
• Sign Gradle CLA
• [N/A] Link to Design Spec for changes that affect more than 1 public API (that is, not in an internal package) or updates to > 20 files
• [N/A] Provide integration tests (under <subproject>/src/integTest) to verify changes from a user perspective
• [N/A] Provide unit tests (under <subproject>/src/test) to verify logic
• [N/A] Update User Guide, DSL Reference, and Javadoc for public-facing changes
• [N/A] Ensure that tests pass locally: ./gradlew quickCheck <impacted-subproject>:check

Gradle Core Team Checklist

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation including proper use of @since and @Incubating annotations for all public APIs
• Recognize contributor in release notes

[jjohannes]

Good catch @szpak! Will be in Gradle 4.1.