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 and ensures this requirement is clearly documented throughout the project. The changes help prevent deployment issues by making sure users are aware of and use the correct CLI version.

Requirements and documentation updates:

• Updated the azure.yaml configuration file to require azd version 1.18.0 or higher, removing support for earlier versions.
• Revised the installation instructions in docs/DeploymentGuide.md to specify azd v1.18.0+ as a prerequisite.
• Added prominent notes in both README.md and docs/DeploymentGuide.md to inform users about the new azd version requirement and provide a direct download link. [1] [2]

Does this introduce a breaking change?

• Yes
• No

How to Test

• Get the code

git clone [repo-address]
cd [repo-name]
git checkout [branch-name]
npm install