fix(css): use charset: 'utf8' by default for css

[sapphi-red]

Description

#10753 changed the default value of charset option passed to esbuild when transforming JS.
This PR changes it for CSS.

Since JS was transformed with charset: 'utf8' already, I think it's fine to change this in a minor.

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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