images: Bump aws-iam-authenticator to 5.21

Bumping aws-iam-authenticator to the latest minor.
This will also bump the image to Alpine 3.19.0

Signed-off-by: Arnaud Meukam <[email protected]>

Author: ameukam

images/git-custom-k8s-auth/cloudbuild.yaml

@@ -20,4 +20,4 @@ steps:
     - gcr.io/$PROJECT_ID/git-custom-k8s-auth:latest
 substitutions:
   _GIT_TAG: '12345'
-  _AWS_IAM_AUTHENTICATOR_VERSION: '0.5.12'
+  _AWS_IAM_AUTHENTICATOR_VERSION: '0.5.21'