Use UTF-8 by default for JSON multipart content in `ContentRequestMatchers`

[azdanov]

This should resolve https://github.com/spring-projects/spring-framework/issues/31923

I've added default UTF-8 charset to be used in DiskFileItemFactory.

This should allow ContentRequestMatchers to correctly handle multipartData which contains unicode.

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[azdanov]

I agree, but I don't have time now. So feel free to add.

[rstoyanchev]

Moving to 6.2 to avoid any regressions in a maintenance release.