SI-8265 Restore 2.10 variance behavior under -Xsource:2.10

Issue deprecation warning under -Xsource:2.10 so time
travelers can have an authentic deprecation experience before
finding that their unsound code no longer compiles in 2.11.
The relevant ticket to the soundness issue is SI-6566.
Merge pull request #3623 from paulp/pr/source-210

SI-8402 Restore 2.10 variance behavior under -Xsource:2.10