Upgrade to 1.21

Author: RebeccaMahany

.circleci/config.yml

@@ -2,7 +2,7 @@ version: 2
 jobs:
   build-go1.20:
     docker:
-        - image: golang:1.20
+        - image: golang:1.21
     working_directory: /go/src/github.com/kolide/kit
     steps: &steps
       - checkout
@@ -13,4 +13,4 @@ workflows:
   version: 2
   build:
     jobs:
-      - build-go1.20
+      - build-go1.21

go.mod

@@ -1,6 +1,6 @@
 module github.com/kolide/kit
 
-go 1.20
+go 1.21
 
 require (
 	github.com/alecthomas/template v0.0.0-20160405071501-a0175ee3bccc