stdlib 6.0.0 is due to be released soon.
This module does not use the updated `merge` function.

(Also bumped minimum version to 4.13.0 as that's the version
that introduced the `Stdlib::Absolutepath` type.)

See
https://github.com/puppetlabs/puppetlabs-stdlib/blob/217068f97edd2396ce37ac8c1bd7e23da621d6be/CHANGELOG.md#supported-release-600
https://tickets.puppetlabs.com/browse/MODULES-9017