docs(remix-config): Update default serverModuleFormat

[jvzaniolo]

As mentioned in the release notes for v2:

• Changed the default serverModuleFormat from cjs to esm (feat!: default to serverModuleFormat esm #6949)

This PR updates the docs accordingly.

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[remix-cla-bot]

Thank you for signing the Contributor License Agreement. Let's get this merged! 🥳

[jvzaniolo]

Duplicate of #7191