all: test with Go 1.16 in CI (#2822)

Also, update examples in README.rst to use 1.16.

For #2820

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 54dd872..b552ec5 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -6,7 +6,7 @@
 
 go_rules_dependencies(is_rules_go = True)
 
-go_register_toolchains(version = "1.16rc1")
+go_register_toolchains(version = "1.16")
 
 http_archive(
     name = "com_google_protobuf",