By default, Nobara keeps the most recent 3 versions of the kernel, and a rescue kernel. This way if a kernel update fails, you can still boot into the previous working kernel.
You can run the following command to delete older kernel versions except the current running one and the last one:
# dnf remove $(dnf repoquery --installonly --latest-limit=-2 -q)
In addition to this, you can also change /etc/dnf/dnf.conf file to specify that you want to keep only last 2 versions of kernel. In this case, yum will automatically retain only the latest kernel and the one before, and delete the rest, every time it upgrades your kernel:
installonly_limit=2 #set kernel count
Save and close the file.