https://github.com/mongodb-js/mongosh/blob/ea0da2dd9b9b4930f84ba41f4a0452aeca18f07c/packages/cli-repl/src/mongosh-repl.ts#L647

prints a warning in Node.js 21, because we are defining .eval for async repls as an async function (for convenience, so that we can use async/await inside the function):

https://github.com/mongodb-js/mongosh/blob/ea0da2dd9b9b4930f84ba41f4a0452aeca18f07c/packages/cli-repl/src/async-repl.ts#L100

We should just ensure that the overridden .eval does not actually return a Promise.