Try to remove kotlinpoet from the dependencies

[Lysander]

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We should investigate, whether we can drop kotlinepoet from our dependencies to upgrade faster to new Kotlin versions.

Currently we cannot upgrade due to kotlinepoet due to #934.

As we only use it for our automatic lenses generation and those code snippets are not too long or complex, we could try to handcraft those with native string templates. If the readbility suffers not too much, I would like to drop the kotlinpoet usage.

As we have quite a good test coverage on this part, this should be not dangerous at all.