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

[Vamshi-Microsoft]

Purpose

This pull request updates the required version of the Azure Developer CLI (azd) for deploying the solution accelerator. All documentation and configuration references to azd have been updated to require version 1.18.0 or higher, and additional notes have been added to help users ensure they have the correct version installed.

Updates to required tooling and documentation:

• Increased the minimum required version of azd to 1.18.0 in the azure.yaml configuration file, removing support for earlier versions.
• Updated all documentation references in docs/DeploymentGuide.md to specify azd version 1.18.0 or higher, replacing the previous 1.15.0+ requirement.
• Added prominent notes in both the 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