go.mod: update grpc-gateway to v2

[jeffswenson]

Attempting to build grpc-gateway v1 using the latest version of io_bazel_go_rules fails due to a breaking change in how io_bazel_go_rules resolves googleapi protobufs (https://github.com/bazelbuild/rules_go/releases/tag/v0.41.0).

Version v2.15.2 was chosen because it is a relatively recent version that occurs before the breaking change was fixed in grpc-gateway. This allows cockroachdb to import the change without bumping the io_bazel_rules_go version. Other repositories that want to bump their io_bazel_rules_go can explicitly depend on a version of grpc-gatewy that includes the bazel fix.