Simplify the syntax of an OAuth secret

[djannot]

Is your feature request related to a problem? Please describe.
When creating an OAuth secret using kubectl, the value is a base64 encoding of the following YAML:

client_secret: secretvalue

Describe the solution you'd like
It's confusing and would be simpler if the value was simply a base64 encoding of the secret value.

[wilmardo]

This would change would be inline with this change already done for the API keys:
#3473

[marcogschmidt]

Whoever picks up this task should also verify whether they can reproduce the error on the TLS secret reported here: #3652.

[github-actions]

This issue has been marked as stale because of no activity in the last 180 days. It will be closed in the next 180 days unless it is tagged "no stalebot" or other activity occurs.