chore(go): Update golang version

Update golang version to 1.21

Signed-off-by: Oluwole Fadeyi <oluwole.fadeyi@venafi.com>

Author: Oluwole Fadeyi

go.mod

@@ -1,6 +1,6 @@
 module github.com/jetstack/preflight
 
-go 1.18
+go 1.21
 
 require (
 	github.com/Jeffail/gabs/v2 v2.7.0