Issue with Deployment on Vercel and Netlify Due to Incompatibility with Node.js Version 20.x

[TomoyaKuroda]

Issue Description:

During the deployment process on Vercel, I found that the default Node.js version (20.x) does not support the required functionality. This incompatibility is hindering the successful deployment of the application.

Proposed Solution:

To address this issue, I'd like to create a new pull request to include instructions for deploying ServerlessWP. These instructions should advise users to utilize Node.js version 18.x to ensure compatibility and successful deployment.

Proposed Pull Request Content:

## Deploying ServerlessWP

5. Please ensure that Node.js version 18.x is selected to deploy ServerlessWP.

Additional Notes

Downgrading version to 18.x is a temporal solution so we need to find a way to make this still work with Node.js 20.x.