go.mod: bump go to 1.23

Currently, go 1.23 is the "old" supported version, meaning the older
versions are no longer supported.

Let's use it as a minimally required version.

Signed-off-by: Kir Kolyshkin <[email protected]>

Author: kolyshkin

.github/workflows/go.yml

@@ -14,7 +14,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        go: ['1.12.x', '1.23.x', '1.24.x']
+        go: ['1.23.x', '1.24.x']
     steps:
       - run: sudo apt-get -qq update
       - name: Install libsystemd-dev

go.mod

@@ -1,5 +1,5 @@
 module github.com/coreos/go-systemd/v22
 
-go 1.12
+go 1.23
 
 require github.com/godbus/dbus/v5 v5.1.0