[Firebase AI] Add initial CHANGELOG entry and restore VertexAI CHANGELOG

[andrewheard]

• Restored the FirebaseVertexAI changelog for history.
• Updated the FirebaseAI changelog to include information about the new SDK.

This is a potential alternative to #14804.

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[paulb777]

Consider keeping these here with an additional note, since we'll probably delete the other directory at 12.0.0

[andrewheard]

Sounds reasonable to me. Any suggestions for the note? And do you know if this will cause any issues with @morganchen12's release note generation?