Bump minimum Go version to 1.13

1.12 is now EOL for almost 2 years: https://endoflife.date/go.

As a reference, supported Ubuntu versions use either 1.10 or 1.13:
https://packages.ubuntu.com/search?keywords=golang-go

So I think this bump is reasonable.

This is done to get access to stdlib methods like errors.Is and
formatting statement %w, which is de facto standard for handling errors
nowadays.

Closes coreos#391

Signed-off-by: Mateusz Gozdek <mgozdek@microsoft.com>

.github/workflows/go.yml

@@ -11,7 +11,7 @@ permissions:
 
 env:
   # Minimum supported Go toolchain
-  ACTION_MINIMUM_TOOLCHAIN: "1.12"
+  ACTION_MINIMUM_TOOLCHAIN: "1.13"
 
 jobs:
   build:

README.md

@@ -1,7 +1,7 @@
 # go-systemd
 
 [![godoc](https://img.shields.io/badge/godoc-reference-5272B4)](https://pkg.go.dev/mod/github.com/coreos/go-systemd/v22/?tab=packages)
-![minimum golang 1.12](https://img.shields.io/badge/golang-1.12%2B-orange.svg)
+![minimum golang 1.13](https://img.shields.io/badge/golang-1.13%2B-orange.svg)
 
 
 Go bindings to systemd. The project has several packages:

go.mod

@@ -1,5 +1,5 @@
 module github.com/coreos/go-systemd/v22
 
-go 1.12
+go 1.13
 
 require github.com/godbus/dbus/v5 v5.0.4