ci: update issue templates to use GitHub alert (google-gemini#3167)

Author: jackwotherspoon

.github/ISSUE_TEMPLATE/bug_report.yml

@@ -5,7 +5,10 @@ body:
   - type: markdown
     attributes:
       value: |
-        Thanks for taking the time to fill out this bug report! Please search [existing issues](https://github.com/google-gemini/gemini-cli/issues) to see if an issue already exists for the bug you encountered.
+        > [!IMPORTANT]  
+        > Thanks for taking the time to fill out this bug report!
+        >
+        > Please search **[existing issues](https://github.com/google-gemini/gemini-cli/issues)** to see if an issue already exists for the bug you encountered.
 
   - type: textarea
     id: problem

.github/ISSUE_TEMPLATE/feature_request.yml

@@ -5,7 +5,10 @@ body:
   - type: markdown
     attributes:
       value: |
-        Thanks for taking the time to suggest an enhancement! Please search [existing issues](https://github.com/google-gemini/gemini-cli/issues) to see if a similar feature has already been requested.
+        > [!IMPORTANT]  
+        > Thanks for taking the time to suggest an enhancement!
+        >
+        > Please search **[existing issues](https://github.com/google-gemini/gemini-cli/issues)** to see if a similar feature has already been requested.
 
   - type: textarea
     id: feature