go.mod: update to Go 1.18

Fedora 35 support was dropped, so we can update to a newer Go.

Stable RHEL 8 and 9 and Fedora 36 ships Go 1.18, so let's switch to it.

"//go:build" directives are now apparently enforced by go fmt, so that's why
there were added.

Also, all the github actions were adjusted to use Go 1.18.

Signed-off-by: Ondřej Budai <ondrej@budai.cz>

cmd/osbuild-service-maintenance/db_test.go

@@ -1,3 +1,4 @@
+//go:build integration
 // +build integration
 
 package main
@@ -8,8 +9,9 @@ import (
 	"testing"
 
 	"github.com/google/uuid"
-	"github.com/osbuild/osbuild-composer/pkg/jobqueue/dbjobqueue"
 	"github.com/stretchr/testify/require"
+
+	"github.com/osbuild/osbuild-composer/pkg/jobqueue/dbjobqueue"
 )
 
 const url = "postgres://postgres:foobar@localhost:5432/osbuildcomposer"