Use more portable invocation of base64 tool

[ayushpatil2122]

Description

kubectl get csr my-svc.my-namespace -o json | \
  jq '.status.certificate = "'$(base64 ca-signed-server.pem | tr -d '\n')'"' | \
  kubectl replace --raw /apis/certificates.k8s.io/v1/certificatesigningrequests/my-svc.my-namespace/status -f -

During the "Upload the signed certificate" step (https://kubernetes.io/docs/tasks/tls/managing-tls-in-a-cluster/#upload-the-signed-certificate) the command didn't work without the "-i" option using the base64 command.

it had to be

kubectl get csr my-svc.my-namespace -o json | \
  jq '.status.certificate = "'$(base64 -i ca-signed-server.pem | tr -d '\n')'"' | \
  kubectl replace --raw /apis/certificates.k8s.io/v1/certificatesigningrequests/my-svc.my-namespace/status -f -

Issue

#48586

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[thisisharrsh]

Deployment preview :: https://deploy-preview-48591--kubernetes-io-main-staging.netlify.app/docs/tasks/tls/managing-tls-in-a-cluster/#upload-the-signed-certificate

[thisisharrsh]

As mentioned in the comment (#48586 (comment)), It will be better if we could add a note. 

[sftim]

/retitle Use more portable invocation of base64 tool

[stmcginnis]

With the BSD version of base64 included with macOS, -i means "input file". It does not take a positional input file argument.
With the gnu version in most linux distros, -i is short for --ignore-garbage. It does allow for a positional input file argument.

There shouldn't be any "garbage" in the pem file, but it looks safe to use it to be compatible with either implementation.

[sftim]

• We could add a comment (shell comment) to explain what's going on here. Otherwise, another contributor may come and "fix" it.
• We could use tabs for "Linux", "macOS" and "Windows" with the appropriate commands to run.