docs: Updated README, azure.yml for minimum azd version 1.18.0

[Vamshi-Microsoft]

Purpose

This pull request updates the minimum required version of the Azure Developer CLI (azd) from 1.15.0 to 1.18.0 throughout the documentation and configuration files. It also adds clearer guidance to users about this requirement in the deployment instructions.

Minimum azd version requirement updates:

• Increased the minimum required version of azd from 1.15.0 to 1.18.0 in the azure.yaml configuration file.
• Updated the version requirement for azd in the tool prerequisites list in docs/DeploymentGuide.md.

Documentation improvements:

• Added a prominent note in the README.md quick deploy section to inform users that azd version 1.18.0 or higher is required, including a download link.
• Added a similar note in the deployment steps of docs/DeploymentGuide.md to remind users to verify their azd version before proceeding.

Does this introduce a breaking change?

• Yes
• No

Golden Path Validation

• I have tested the primary workflows (the "golden path") to ensure they function correctly without errors.

Deployment Validation

• I have validated the deployment process successfully and all services are running as expected with this change.

[github-actions]

🎉 This PR is included in version 1.7.0 🎉

The release is available on GitHub release

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