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>

internal/client/integration_test.go

@@ -1,6 +1,7 @@
 // Package client - integration_test contains functions to setup integration tests
 // Copyright (C) 2020 by Red Hat, Inc.
 
+//go:build integration
 // +build integration
 
 package client