We are currently returning a 3.0 context for Vulkan backends (issue
3425).   However, we aren't quite 3.0 conformant yet, so this bug
will drop that back down to 2.0 in getMaxConformantESVersion().

Bug: angleproject:3734
Test: dEQP
Change-Id: I2c5a6ee0ddb3bd52fdc92fdc9b12cf3c6d8e0510
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1716244
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>