docs: modify to conform to the API of webpack-dev-server v5

[kawamataryo]

Description

The before option was based on webpack-dev-server v3, so I updated it to the latest v5 API.

https://webpack.js.org/configuration/dev-server/#devserversetupmiddlewares

Additional context

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What is the purpose of this pull request?

• [ ] Bug fix
• [ ] New Feature
• [x] Documentation update
• [ ] Other

Before submitting the PR, please make sure you do the following

• [x] Read the Contributing Guidelines.
• [x] Read the Pull Request Guidelines and follow the Commit Convention.
• [x] Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• [x] Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).