Manage kotlin version to work around RequireUpperBoundDeps failure (#504)

[jonaslind]

As a workaround until jenkinsci/plugin-pom#705 is fixed and propagated, the kotlin version is explicitly set to match what okhttp-api-plugin provides.

This prevents a RequireUpperBoundDeps failure to be triggered for different kotlin versions in the okhttp-api-plugin dependency subtree.

More information in the related issue:
#504

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[timja]

thanks!