use VERSION_ID from os-release

[brycekahle]

VERSION_ID is more appropriate for scripts and other usages, since VERSION can contain spaces and codenames

from os-release manpage:

VERSION=
           A string identifying the operating system version, excluding
           any OS name information, possibly including a release code
           name, and suitable for presentation to the user. This field
           is optional.

           Examples: "VERSION=17", "VERSION="17 (Beefy Miracle)"".

 VERSION_ID=
           A lower-case string (mostly numeric, no spaces or other
           characters outside of 0–9, a–z, ".", "_" and "-") identifying
           the operating system version, excluding any OS name
           information or release code name, and suitable for processing
           by scripts or usage in generated filenames. This field is
           optional.

           Examples: "VERSION_ID=17", "VERSION_ID=11.04".

[shirou]

When I first looked at this PR, I thought that this change would have a very broad impact.

However, after reading the comments in the linked PR, I realized that it wouldn't really matter, since the priority for os-release is lower than for files like lsb-release and distribution files(e.g. debian_version).

One thing to note is that there is an issue #1576. In this case, os-release has the highest priority, so the impact will be significant. However, I approve this PR because I think this version value should actually return the VERSION_ID.

Your contribution is very much appreciated.