This change updates the gtest-version used by Bazel.
Also, `//external:{gtest,gtest_main}` is deprecated so we can remove some
of the uses of the discouraged `bind` function.

RELNOTES[bazel]: Starting with Protobuf 3.13.0, building and running
Protobuf tests requires `@com_google_googletest//:{gtest,gtest_main}`
instead of `//external:{gtest,gtest_main}`. Use
`--@com_google_protobuf//:incompatible_use_com_google_googletest=true`
to verify your workspace is not affected by this change.