Prepare 6.0.0 release

[wing328]

Prepare v6.0.0 release

cc @OpenAPITools/generator-core-team

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[sjoblomj]

I do not want to be a pest about it, but might I ask if #12377 can be included in 6.0.0 (or if there is anything missing in the PR or otherwise an issue with it)?

It is needed to have the springdoc migration feature complete for Kotlin Spring, without it the work is just short of being complete.

[wing328]

Usually we don't do last minute merge but looks like that one is better included in v6.0.0 as well.