Require Go >= 1.16

Go 1.15 is not supported since Go 1.17 release (16 Aug 2021), and some
packages that we use already require Go 1.16+ (notably,
github.com/cilium/ebpf v0.7.0).

Let's require Go 1.16+.

Remove Go version requirement from README when describing dependencies,
since it is no longer needed:

	$ GO=go1.15.15 make vendor
	go1.15.15 mod tidy
	go mod tidy: go.mod file indicates go 1.16, but maximum supported version is 1.15
	make: *** [Makefile:141: vendor] Error 1

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

.github/workflows/test.yml

@@ -21,7 +21,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        go-version: [1.15.x, 1.16.x, 1.17.x]
+        go-version: [1.16.x, 1.17.x]
         rootless: ["rootless", ""]
         race: ["-race", ""]
         criu: [""]

README.md

@@ -23,7 +23,7 @@ A third party security audit was performed by Cure53, you can see the full repor
 
 ## Building
 
-`runc` only supports Linux. It must be built with Go version 1.15 or higher.
+`runc` only supports Linux. It must be built with Go version 1.16 or higher.
 
 In order to enable seccomp support you will need to install `libseccomp` on your platform.
 > e.g. `libseccomp-devel` for CentOS, or `libseccomp-dev` for Ubuntu
@@ -110,7 +110,7 @@ You can run a test using your container engine's flags by setting `CONTAINER_ENG
 
 `runc` uses [Go Modules](https://github.com/golang/go/wiki/Modules) for dependencies management.
 Please refer to [Go Modules](https://github.com/golang/go/wiki/Modules) for how to add or update
-new dependencies. When updating dependencies, be sure that you are running Go `1.14` or newer.
+new dependencies.
 
 ```
 # Update vendored dependencies

go.mod

@@ -1,6 +1,6 @@
 module github.com/opencontainers/runc
 
-go 1.15
+go 1.16
 
 require (
 	github.com/checkpoint-restore/go-criu/v5 v5.1.0