Add option to use the experimental Gradle builtin dependency parser

What does this MR do?

This merge request adds a new DS_EXPERIMENTAL_GRADLE_BUILTIN_PARSER environment variable to gradle.Flags and represents a small iteration towards Replace gemnasium-gradle-plugin with the htmlDe... (gitlab-org&12361 - closed).

What are the relevant issue numbers?

Add initial support for DS_EXPERIMENTAL_GRADLE_... (gitlab-org/gitlab#437268 - closed) • Philip Cunningham • 16.9 • On track

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
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• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer