Phase out support for kernel v5.10

We would like to drop support for Linux v5.10 in the future. It is projected to reach end of life December 2026 ([link](https://www.kernel.org/category/releases.html)).

Note that this will not affect currently supported releases (v1.13, v1.14) nor the upcoming v1.15 release. At a later point, we will confirm the first version that will drop support. After support is phased out, Firecracker will no longer officially support Linux v5.10, and it will be removed from our tested configurations. The new minimal supported kernel version will become v6.1.