Kestrel is part of the .net core runtime and no longer a separate repo. It’s patched when you install a .net runtime patch.
Asp.net core follows the node model. There a long term support (LTS) releases good for a little over 2 years, and minor release good for 3 months after the next release. There is a new version every year. 3.1 is the current LTS, and 6.0 will be the next due soon.