feat(generative-ai): Update Gemini model versions (#11795)

## Description

Fixes 

- Update the Gemini 1.0 vision model with the 1.5 Flash model.

## Checklist
- [x] **Tests** pass:   `nox -s py-3.9` (see [Test Environment Setup](https://togithub.com/GoogleCloudPlatform/python-docs-samples/blob/main/AUTHORING_GUIDE.md#test-environment-setup))
- [x] **Lint** pass:   `nox -s lint` (see [Test Environment Setup](https://togithub.com/GoogleCloudPlatform/python-docs-samples/blob/main/AUTHORING_GUIDE.md#test-environment-setup))
- [x ] Please **merge** this PR for me once it is approved

generative_ai/gemini_multi_image_example.py

@@ -38,7 +38,7 @@ def generate_text_multimodal(project_id: str) -> str:
         mime_type="image/png",
     )
 
-    model = GenerativeModel(model_name="gemini-1.0-pro-vision-001")
+    model = GenerativeModel(model_name="gemini-1.5-flash-001")
     response = model.generate_content(
         [
             image_file1,

generative_ai/gemini_pro_basic_example.py

@@ -24,7 +24,7 @@ def generate_text(project_id: str) -> None:
 
     vertexai.init(project=project_id, location="us-central1")
 
-    model = GenerativeModel(model_name="gemini-1.0-pro-vision-001")
+    model = GenerativeModel(model_name="gemini-1.5-flash-001")
 
     image_file = Part.from_uri(
         "gs://cloud-samples-data/generative-ai/image/scones.jpg", "image/jpeg"

generative_ai/gemini_single_turn_video_example.py

@@ -24,7 +24,7 @@ def generate_text(project_id: str) -> str:
 
     vertexai.init(project=project_id, location="us-central1")
 
-    vision_model = GenerativeModel(model_name="gemini-1.0-pro-vision-001")
+    vision_model = GenerativeModel(model_name="gemini-1.5-flash-001")
 
     # Generate text
     response = vision_model.generate_content(