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/.gitignore b/.gitignore
index 6d07751..804cfba 100644
--- a/.gitignore
+++ b/.gitignore
@@ -272,6 +272,7 @@
 /lib/erl/logs/
 /lib/go/pkg
 /lib/go/src
+/lib/go/test/fuzz/gopathfuzz
 /lib/go/test/gopath/
 /lib/go/test/ThriftTest.thrift
 /lib/nodets/test-compiled/