[hardknott] CHANGELOG: Merge 8.X release information into hardknott

[amstewart]

It's getting to be burdensome to cherry-pick changes between the hardknott and sumo mainlines, and also to their release branches. We missed integrating some 8.X release information into hardknott already, and I think it will be confusing for users to determine which changelog they should be looking at.

So I'd like to adopt a policy of only using the latest mainline (in this case, "hardknott") as the canonical CHANGELOG and having all other branches just reference the github URL for the main changelog, rather than keeping their own copies.

https://github.com/ni/nilrt/blob/HEAD/CHANGELOG.md

As a first step, this PR merges the missing 8.X release information into the hardknott changelog, and also adds release information for 8.13.

Next, I'll add a line to the top of the sumo changelogs to direct users to the canonical changelog.

[amstewart]

Patch V2

• Rebased on latest nilrt/master/hardknott ref to pull other changelog updates.
• Added a short description to the 8.13 release - as we did with the 8.12 release. @mike-petersen-ni