Where can I find out when the next kernel version will be released for my distro version?

Kernel version X.YY do not change by default for installed Ubuntu

Except for LTS HWE installations. (See below)

No one knows which future version of Ubuntu will get which future version of the kernel

Short Term Supported versions of Ubuntu

Short term support versions of Ubuntu such as 18.10 are supported only for 9 months after the day of their release. They don't get any new kernel version. For example, Ubuntu 18.10 was released with kernel 4.18 and it will remain at that level till its end of life.

Security and (some) bug fixes are backported from newer kernels to the supported kernel. That is, as long as Ubuntu 18.10 is supported it will continue to get updates for kernel 4.18 which will incorporate some of the benefits of newer kernels.

Long Term Supported (LTS) versions of Ubuntu

LTS versions of Ubuntu such as 18.04 are supported for 5 years minimum. The version of the kernel it came with will be updated with security fixes for that time. For example, Ubuntu 18.04 started out with kernel 4.15. If you installed Ubuntu 18.04 when it was released, then by default it will remain at kernel 4.15.

The main purpose of making newer versions of kernel available for the LTS version of Ubuntu is to support new hardware that comes out after the initial release of the LTS version of Ubuntu. When Ubuntu 18.04.2 was released earlier this month the new ISO came with kernel 4.18, the same kernel Ubuntu 18.10 comes with. There were no kernel 4.16, or 4.17 for Ubuntu 18.04.2.

Almost all the features of the existing installations of Ubuntu 18.04 and 18.04.1 were updated to 18.04.2 earlier this month except the new kernel 4.18 and drivers that support newer graphics cards. There is a way to update existing installation of Ubuntu 18.04 to kernel 4.18. This is described here.

LTS HWE Exception

Those who installed Ubuntu 18.04.2 LTS directly from the ISO image or enabled HWE manually in 18.04 or 18.04.1 installations will continue to get kernel updates associated with 18.04.3, 18.04.4, 18.04.5, and 18.04.6 as they are released.

Your 3rd link has all the information for the future

Look for a graph titled 18.04.x Ubuntu Kernel Support Schedule. You may have to zoom in to see the dates. Ubuntu 18.04.x LTS will not get all the kernels that will be released. In particular it will only get the kernels that will be built for various "Short Term Releases".

In August of 2019, Ubuntu 18.04.3 will be released. It will get the same kernel Ubuntu 19.04 gets when it released in April of 2019. The exact kernel version is not decided yet. It will depend on the stable kernel at the time of Ubuntu 19.04 development kernel freeze on April 1, 2019. Around April 1, the Ubuntu development team will decide based on which stable kernel versions are available and how much work needs to be done to customize that version for Ubuntu, the actual kernel version that will go into Ubuntu 19.04. This decision will in turn determine the kernel version number for Ubuntu 18.04.3 that will be released in August, 2019. The next version of the kernel used in Ubuntu may be 4.19, 4.20, 5.0, 5.1, or 5.2 or higher. Watch out for an announcement from Canonical on or around April 1, 2019.

Based on the kernel of Ubuntu 19.04, the Ubuntu 18.04.3 LTS has kernel 5.0

This is why the graph shows vTBD in place of kernel version.

Hope this helps