Pinned composer dependency to rector 0.17.12

[cmuench]

With the last release v32 the dependency to rector was changed from ^0.15.10 to 0.17.12. To pin the dependency to a fixed version is a bad practice in the PHP world.

It's better to define it at least as ~0.17.12 to allow bug fix releases of rector.
In the meantime a 0.17.3 of rector is released which cannot be updated in a Magento project.

[m2-assistant]

Hi @cmuench. Thank you for your report.
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Add a comment to assign the issue: @magento I am working on this

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