Closes#43040
Release Notes:
- Remove the end-of-support Gemini 1.5 model from the options.
- Remove the older Gemini 2.0 model from the options.
- Please let me know if you think it's better to keep it, as it is still
a usable model.
- Update the incorrect amounts for some input/output tokens.
- Update the default model to Gemini 2.5 Flash-Lite.
- Rename variant `Gemini3ProPreview` to `Gemini3Pro`
When this PR is merged, users will be able to select the following
Gemini models.
- 2.5 Flash
- 2.5 Flash-Lite
- 2.5 Pro
- 3 Pro
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