Update getMaxConformantESVersion() to support GLES 3.1.

This requires skipping some tests on Nvidia due to XFB failures.

dEQP-GLES3.functional.transform_feedback.array_element.interleaved.*
dEQP-GLES3.functional.transform_feedback.array_element.separate.*
dEQP-GLES3.functional.transform_feedback.array.interleaved.*
dEQP-GLES3.functional.transform_feedback.array.separate.*
dEQP-GLES3.functional.transform_feedback.random.*

Bug: angleproject:3520
Test: CQ
Change-Id: I3de0e8f7a9e9c6e800be9eb2c04e713e1f88fc08
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2213810Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Tim Van Patten <timvp@google.com>