Fix version comparisons for ansible_distribution_major_version

Comparisons like ansible_distribution_major_version > "7" were broken.
Comparing number strings of different length may result in an unexpected result.
For example "10" > "7" evaluates to false.

Comparisons like ansible_distribution_major_version | int > 7 were fine.

... is version('7', '>') could also be used to compare versions. As the
... | int conversion is used in many other places, use it for consistency.