After restoring the disk and starting over, I issued grub2-mkconfig --output=/boot/efi/EFI/rocky/grub.cfg before rebooting, and all was well. The machine boots normally now.
Adding that step to the end of the upgrade procedure:
rpm -e --nodeps centos-gpg-keys centos-linux-release centos-linux-repos
rpm -ivh
https://download.rockylinux.org/pub/rocky/8.3/BaseOS/x86_64/os/Packages/rocky-release-8.3-11.el8.noarch.rpm
https://download.rockylinux.org/pub/rocky/8.3/BaseOS/x86_64/os/Packages/rocky-repos-8.3-11.el8.noarch.rpm
https://download.rockylinux.org/pub/rocky/8.3/BaseOS/x86_64/os/Packages/rocky-gpg-keys-8.3-11.el8.noarch.rpm
dnf distro-sync -y
grub2-mkconfig --output=/boot/efi/EFI/rocky/grub.cfg
REBOOT