treewide: update to go 1.21.4

Change-Id: I4f4cdaf696a13dd967834ec5ea690aa266e3aea8
Reviewed-on: https://review.monogon.dev/c/monogon/+/2310
Tested-by: Jenkins CI
Reviewed-by: Leopold Schabel <leo@monogon.tech>
diff --git a/go.mod b/go.mod
index fcd887c..0900979 100644
--- a/go.mod
+++ b/go.mod
@@ -1,6 +1,6 @@
 module source.monogon.dev
 
-go 1.18
+go 1.21
 
 // Kubernetes is not fully consumable as a module, fix that
 replace (