feat(ai): add deprecation notices to Imagen types

[daymxn]

Per b/496563474,

This PR adds deprecation notices to all of the imagen related APIs, to prepare for removal of said APIs in June.

@available markers have been added for each related type, with a descriptive message of what's going on, and how to migrate. Although, since we can't provide links within the annotation message, a matching @DeprecationSummary has also been included for each marker.

Some minor notes:

• The positioning of @DeprecationSummary may appear to be odd in a few of the usages. This is due to the way xcode renders the notice. If it's not put after the first summary, then xcode won't render anything else (such as the arguments, or the discussion portion).
• The @available markers still include a message, as this is the message that's actually used in the IDE for warnings. The one in the @DeprecationSummary is instead used in the generated docs.
• Some internal types also have had these markers added. Even though these aren't public facing, it avoids a few warning messages from popping up in our build system. It also maintains consistency, and makes things a bit easier to audit.

#ai-logic-imagen-shutdown

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[andrewheard]

Please replace imagen with Imagen throughout.

[andrewheard]

@daymxn Would you mind merging from main to verify that the tests are fixed?

[andrewheard]

Thanks, @daymxn!

[rachelsaunders]

In each of the messages, is it possible to add the link to the migration guide? I just want to make sure folks don't have to go hunting for that info if they see one of these deprecation messages, ya know?

[rachelsaunders]

Suggested change

	/// (the "Nano Banana" models).](https://firebase.google.com/docs/ai-logic/imagen-models-migration)
	/// (the "Nano Banana" models)](https://firebase.google.com/docs/ai-logic/imagen-models-migration).

IF you spin up a new PR to address my high-level comment that I just posted, could you pls make this change throughout? It's just a nit, so if you're NOT spinning up a new PR, then don't worry about it at all.