Add warning about undocumented authentication methods

Context

Because of the way code has been reused to implement authentication for the Package Registry, there are a number of undocumented authentication methods for the various package formats. Just because these exist does not mean that customers should use them, much less depend on them being supported. We should add a warning to the documentation that these undocumented authentication methods should not be used and may be removed in the future.

What does this MR do?

For each of the Package Registry package formats, add a warning to the documentation that undocumented authentication methods should not be used and may be removed in the future.

For package format documentations that have an Authentication section, I added the warning inside that section, right after the list of supported authentication methods.

For documentations that do not have an Authentication section, I added the warning at the topmost section.

Related issues

#326670 (closed)

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