THRIFT-5447: Update supported Go versions

Client: go

Update go versions used in travis to 1.16.7 and 1.17, update
LANGUAGES.md, and update go's README to clarify on support policy.

This change will be cherry-picked into 0.15.0 branch after merged.
diff --git a/go.mod b/go.mod
index 474ec99..d620e5e 100644
--- a/go.mod
+++ b/go.mod
@@ -1,5 +1,5 @@
 module github.com/apache/thrift
 
-go 1.15
+go 1.16
 
 require github.com/golang/mock v1.5.0