Upgrade from LM (Cinnamon) 21.3 to 22 Fails: Unbootable; boot-repair Does Not Fix; grub-efi-amd64-signed Not Installed

Multiple attempts to use mint-upgrade to upgrade from Linux Mint (Cinnamon) 21.3 to 22 failed with the system unbootable. Use of boot-repair from a Linux Mint Live environment did not fix the problem. After restoring to a working LM 21.3 via Timeshift, ``` apt-get upgrade apt-get dist-upgrade ``` also produced an unbootable system, but revealed that grub-efi-amd64-signed and grub-efi-amd64-bin had not been installed. In the attempted upgrade using `apt-get upgrade` and `dist-upgrade`, those packages had been held back due to phased updates. Forcing installation of grub-efi-amd64-signed (bringing grub-efi-amd64-bin in as a dependency) resulted in a bootable system. Some references to LM 21.3 persisted because the LM 22 version of the base-files package had not been installed; LM's versioning causes the LM22 package to be considered a "downgrade". Installing base-files, specifying the LM 22 version of the package: ``` apt-get install base-files=13ubuntu10mint22.0.0+wilma ``` fixed that. I have no idea what else, if anything, was not successfully upgraded.