Development on Ubuntu 21.04 kicked off last year in late October, quickly after the release of Ubuntu 20.10 (Groovy Gorilla), on which the upcoming release was based at the point in time, thus shipping with Linux kernel 5.8.
About a month ago, I told you that Ubuntu 21.04 is powered by Linux kernel 5.10 LTS, and the fact that it would be a good move from Canonical to ship the Hirsute Hippo release with a long-term supported kernel. But, since Ubuntu 21.04 is not an LTS release, why not use a more recent kernel. Therefore, the final Ubuntu 21.04 (Hirsute Hippo) release will be powered by Linux kernel 5.11, which brings lots of hardware improvements and new features. As you can imagine, at this point in its development cycle, Linux 5.11 will be the final kernel to be used in the Ubuntu 21.04 release.