[mdatagen] Do not show "supported pipeline types" for extensions

[atoulme]

Component(s)

No response

Is your feature request related to a problem? Please describe.

See #20232 (comment)

Describe the solution you'd like

Do not show "supported pipeline types" for extensions.

Describe alternatives you've considered

No response

Additional context

No response

[github-actions]

Pinging code owners:

• mdatagen: @dmitryax

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[github-actions]

Pinging code owners for cmd/mdatagen: @dmitryax. See Adding Labels via Comments if you do not have permissions to add labels yourself.