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) to 1.18.0 across documentation and configuration files. It also adds clearer guidance for users to ensure they have the correct version installed before deploying the solution.

Updates to required tooling and documentation:

• Increased the minimum required version of azd from 1.15.0 to 1.18.0 in the azure.yaml configuration file to enforce compatibility.
• Updated all relevant documentation (README.md and docs/DeploymentGuide.md) to specify azd version 1.18.0+ as a requirement, replacing previous references to version 1.15.0. [1] [2]
• Added prominent notes in both the quick deploy section of README.md and the deployment steps in docs/DeploymentGuide.md to remind users to install or upgrade to azd 1.18.0 or higher, including direct download links for convenience. [1] [2]

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.

What to Check

Verify that the following are valid

• I have built and tested the code locally and in a deployed app
• For frontend changes, I have pulled the latest code from main, built the frontend, and committed all static files.
• This is a change for all users of this app. No code or asset is specific to my use case or my organization.

[github-actions]

🎉 This PR is included in version 1.7.0 🎉

The release is available on GitHub release

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