Mark --pod/-p flag of kubectl exec command as deprecated

[quasoft]

What this PR does / why we need it:
Marks the --pod (-p shorthand) flag of kubectl exec command as deprecated.
Hides the flag from the help menu, but shows a message when command is executed with this flag.

Which issue this PR fixes:
Fixes: kubernetes/kubectl#104

This is a remake of PR #54629.

Release note:

Flag --pod (-p shorthand) of kubectl exec command marked as deprecated

[tanshanshan]

/ok-to-test

[quasoft]

/test pull-kubernetes-e2e-kops-aws

[quasoft]

/test pull-kubernetes-e2e-kops-aws

[quasoft]

/test pull-kubernetes-e2e-kops-aws

[apelisse]

That's exactly what we need and it works great. Thank you @quasoft
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: apelisse, quasoft

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[apelisse]

@quasoft I was looking at the original PR, and it was also fixing "port-forward". If you want to follow-up by looking-up all the deprecated flags that are not properly deprecated, that'd be awesome.

[k8s-github-robot]

Automatic merge from submit-queue (batch tested with PRs 66593, 66727, 66558). If you want to cherry-pick this change to another branch, please follow the instructions here.

[quasoft]

@apelisse It seems the port-forward flag was already removed.

[marpaia]

/sig cli