[Core] [UA] Setting severity for API deprecations

[Bamieh]

Should we be blocking upgrade critical severity api deprecations? Or should we allow that? Or should we prevent devs from setting critical severity for api deprecations.

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copying comments from PR thread #199026 (comment)

JL

This actually raises something of an oversight: I don't think ever setting an internal route as deprecated was intended for this API 🤔 , although being able to set critical on it could be useful if we want to remove it and have known integrations.
My gut feel would be to remove it from the types for internal routes (versioned or non-versioned). FWIW I don't think we should block on it here.

Tina

++ internal APIs aren't supposed to be consumed externally and we can add .jsdocs to warn plugin developers that the route is deprecated.

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[jloleysens]

After some discussion I think the likely path forward we settled on was:

• Allow API deprecations to only register as warning for now
• In future, we might enhance this by also having something like breaking so that the UI can "lift" these to user attention

Does this sound right @Bamieh ?