Remove pipeline variables section from azure.yaml and update azd version requirement

[Copilot]

This PR modernizes the azure.yaml configuration by removing the pipeline: variables: section, which is no longer needed as of Azure Developer CLI version 1.17.0. The Azure Developer CLI now auto-detects the needed variables from main.parameters.json.

Changes Made

1. Removed the entire pipeline: block containing 14 variable definitions that are now redundant
2. Updated requiredVersions.azd from >= 1.10.0 to >= 1.17.0 to enforce the minimum version that supports auto-detection
3. Bumped metadata.template version from 0.2.0 to 0.3.0 to indicate this modernization

Before

requiredVersions:
    azd: ">= 1.10.0"
# ... services configuration ...
pipeline:
  variables:
    - AZURE_OPENAI_CHAT_DEPLOYMENT
    - AZURE_OPENAI_CHAT_MODEL
    # ... 12 more variables ...

After

requiredVersions:
    azd: ">= 1.17.0"
# ... services configuration (no pipeline section) ...

All the removed variables are properly defined in infra/main.parameters.json with the ${VARIABLE_NAME=default} syntax, ensuring azd >= 1.17.0 can auto-detect them. The CI workflows continue to work as they define these same variables as GitHub variables/secrets.

Benefits

• Simplified configuration - Variables are now inferred automatically
• Reduced duplication - Single source of truth in main.parameters.json
• Reduced drift risk - No need to maintain variables in multiple places
• Aligned with best practices - Uses modern azd capabilities

Fixes #329.

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