Bump circle-ci image and update README to build using latest go (#5458)

Signed-off-by: Ondřej Benkovský <ondrej.benkovsky@jamf.com>

.circleci/config.yml

@@ -18,7 +18,7 @@ initWorkingDir: &initWorkingDir
 
 integrationDefaults: &integrationDefaults
   machine:
-    image: ubuntu-2004:202201-02
+    image: ubuntu-2004:2022.04.2
   working_directory: ~/go/src/${CIRCLE_PROJECT_USERNAME}/coredns
   environment:
     - K8S_VERSION: v1.22.0

README.md

@@ -73,7 +73,7 @@ CoreDNS requires Go to compile. However, if you already have docker installed an
 setup a Go environment, you could build CoreDNS easily:
 
 ```
-$ docker run --rm -i -t -v $PWD:/v -w /v golang:1.17 make
+$ docker run --rm -i -t -v $PWD:/v -w /v golang:1.18 make
 ```
 
 The above command alone will have `coredns` binary generated.