fix: bump go version to 1.16 (#2678)

Author: ronething

api/go.mod

@@ -1,6 +1,6 @@
 module github.com/apisix/manager-api
 
-go 1.15
+go 1.16
 
 require (
 	github.com/coreos/go-oidc/v3 v3.3.0

docs/en/latest/install.md

@@ -75,7 +75,7 @@ Before using source codes to build, make sure that the following dependencies ar
 
 For `manager-api`:
 
-1. [Golang](https://golang.org/dl/) 1.13+
+1. [Golang](https://golang.org/dl/) 1.16+
 
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