I did:
sudo dpkg --list | egrep -i --color 'linux-image'
sudo apt-get --purge remove linux-image-5.15.0-1-librem5
sudo mv initrd.img-5.13.0-1-librem5 /boot
sudo dpkg --list | egrep -i --color 'linux-image'
sudo apt-get --purge remove linux-image-5.13.0-1-librem5
ls -lh /boot
total 307M
-rw-r--r-- 1 root root 2.3K Jan 23 07:12 boot.scr
-rw-r--r-- 1 root root 2.3K Jan 23 02:18 boot.scr.bak
-rw-r--r-- 1 root root 184K Jan 17 11:33 config-5.16.0-1-librem5
lrwxrwxrwx 1 root root 53 Jan 23 07:12 dtb -> dtbs/5.16.0-1-librem5/freescale/imx8mq-librem5-r4.dtb
lrwxrwxrwx 1 root root 53 Jan 23 07:12 dtb-5.16.0-1-librem5 -> dtbs/5.16.0-1-librem5/freescale/imx8mq-librem5-r4.dtb
drwxr-xr-x 5 root root 1.0K Jan 23 02:17 dtbs
drwxr-xr-x 2 root root 1.0K Jul 13 2021 grub
-rw-r--r-- 1 root root 62M Jan 23 07:12 initrd.img
-rw-r--r-- 1 root root 62M Dec 10 07:55 initrd.img-5.13.0-1-librem5
-rw-r--r-- 1 root root 62M Jan 23 07:12 initrd.img-5.16.0-1-librem5
-rw-r--r-- 1 root root 62M Jan 23 02:18 initrd.img.bak
drwx------ 2 root root 12K Sep 11 07:31 lost+found
-rw-r--r-- 1 root root 4.8M Jan 17 11:33 System.map-5.16.0-1-librem5
-rw-r--r-- 1 root root 24M Jan 23 07:12 vmlinuz
-rw-r--r-- 1 root root 8.5M Jan 17 11:33 vmlinuz-5.16.0-1-librem5
-rw-r--r-- 1 root root 24M Jan 23 02:18 vmlinuz.bak
The /boot remains 76% full and the file initrd.img-5.13.0-1-librem5 which I moved to /boot again, was not purged. I think, I will just move it away again.