It looks like I fixed the broken installation with the following steps:

    Manually removed the broken installation package from linux-image-5.4.88-xanmod1 using synaptic, using the mark for complete removal. This could not be done, but the following recommendation was given.
    $ sudo dpkg --configure -a
    Reboot and manually select kernel 5.4.89-xanmod1
    An attempt was made to manually remove a broken linux installation package-image-unsigned-5.8.0-38- universal, using a synaptic label for complete removal. It had to be replaced with a linux image-5.8.0-38- generic, which failed because it was marked as being in an inconsistent state and recommended for re-installation.
    Using synaptic, a manually labeled linux-image-unsigned package-5.8.0-38- universal for updating. It had to be replaced with a linux image-5.8.0-38- a generic that has been crowned with success.
    $ sudo apt autorun
    I used Grub Customizer to return two entries 5.4.89-xanmod1 back to the top of the "Advanced Options for Ubuntu" submenu list.
    Used Grub Customizer to copy the lines "linux" and "initrd" from the first entry 5.4.89-xanmod1 to the menuentry "Ubuntu", so that 5.4.89-xanmod1 is again the default menu item (i.e. the first one) that Grub2 can find. [I have not been able to find a suitable command line tool for this, but suppose it is updated every time a new kernel is installed in Ubuntu.]
Saved changes using Grub Customizer.