Update Xcode version for Darwin test builds (#122)

Support for Xcode 12.0.0 has been deprecated on CircleCI. This change
updates the build configuration to use the latest stable Xcode version
available.

While testing it was found that the build steps were actually downloading an older
version of Go 1.17 and not the expected latest version. This change explicitly
sets the version of Go to be used for test builds.

There has to be a better wait to do this so we may look to replace this
config at a future time.

.circleci/config.yml

@@ -15,7 +15,7 @@ executors:
     resource_class: medium+
   darwin:
     macos:
-      xcode: "12.0.0"
+      xcode: "13.4.1"
 
 commands:
   install-go-run-tests-unix:
@@ -58,14 +58,14 @@ jobs:
     steps:
       - install-go-run-tests-unix:
           GOOS: darwin
-          GOVERSION: "1.17"
+          GOVERSION: "1.17.13"
   test-windows:
     executor:
       name: win/vs2019
       shell: bash.exe
     steps:
       - install-go-run-tests-windows:
-          GOVERSION: "1.17"
+          GOVERSION: "1.17.13"
   check-lint:
     executor: golang
     resource_class: xlarge