Upgrade go.mod go version to 1.16 (#16764)

Author: wxiaoguang

README.md

@@ -73,7 +73,7 @@ or if sqlite support is required:
 
 The `build` target is split into two sub-targets:
 
-- `make backend` which requires [Go 1.13](https://golang.org/dl/) or greater.
+- `make backend` which requires [Go 1.16](https://golang.org/dl/) or greater.
 - `make frontend` which requires [Node.js 12.17](https://nodejs.org/en/download/) or greater and Internet connectivity to download npm dependencies.
 
 When building from the official source tarballs which include pre-built frontend files, the `frontend` target will not be triggered, making it possible to build without Node.js and Internet connectivity.

go.mod

@@ -1,6 +1,6 @@
 module code.gitea.io/gitea
 
-go 1.14
+go 1.16
 
 require (
 	cloud.google.com/go v0.78.0 // indirect