Update the boilerplate to use serverless environment variables

The project uses serverless-plugin-write-env-vars plugin to work with environment variables. Newer version of serverless supports supports env variables out of the box.

Start by locating the serverless configuration and the reference to serverless-plugin-write-env-vars in the repository. Check how environment variables are currently configured, then update the boilerplate to use the newer Serverless built-in support and verify that the existing environment-variable workflow still works.

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