fix: switch generated code to go sdk 1.21

Author: bahkauv70

templates/go/go.mod.mustache

@@ -1,6 +1,6 @@
 module {{gitHost}}/{{gitUserId}}/{{gitRepoId}}/services{{#isGoSubmodule}}/{{packageName}}{{/isGoSubmodule}}
 
-go 1.18
+go 1.21
 
 require (
 	{{gitHost}}/{{gitUserId}}/{{gitRepoId}}/core v0.15.1