Adopt flattened interface for ChangeRequestSCMHead

[das7pad]

• See Inline ChangeRequestSCMHead2 interface into ChangeRequestSCMHead scm-api-plugin#444

Adopt the flattened interface for ChangeRequestSCMHead (previously ChangeRequestSCMHead2).

Testing done

Run tests locally, they cover all three env vars:

branch-api-plugin/src/test/java/jenkins/branch/BranchNameContributorTest.java

Lines 121 to 123 in ff1f4ee

	assertThat(env.get("CHANGE_ID"), is(cr1Num.toString()));
	assertThat(env.get("CHANGE_TARGET"), is("master"));
	assertThat(env.get("CHANGE_BRANCH"), is("CR-" + cr1Num));

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed