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/build/docker/ubuntu-xenial/Dockerfile b/build/docker/ubuntu-xenial/Dockerfile
index 441b692..3443f92 100644
--- a/build/docker/ubuntu-xenial/Dockerfile
+++ b/build/docker/ubuntu-xenial/Dockerfile
@@ -128,9 +128,9 @@
       libglib2.0-dev
 
 # golang
-ENV GOLANG_VERSION 1.15.10
+ENV GOLANG_VERSION 1.16.7
 ENV GOLANG_DOWNLOAD_URL https://golang.org/dl/go$GOLANG_VERSION.linux-amd64.tar.gz
-ENV GOLANG_DOWNLOAD_SHA256 4aa1267517df32f2bf1cc3d55dfc27d0c6b2c2b0989449c96dd19273ccca051d
+ENV GOLANG_DOWNLOAD_SHA256 7fe7a73f55ba3e2285da36f8b085e5c0159e9564ef5f63ee0ed6b818ade8ef04
 RUN curl -fsSL "$GOLANG_DOWNLOAD_URL" -o golang.tar.gz && \
       echo "$GOLANG_DOWNLOAD_SHA256  golang.tar.gz" | sha256sum -c - && \
       tar -C /usr/local -xzf golang.tar.gz && \