That is not enough information to work with sadly. You have described your
environment but not the failure; and I do not have a linux machine to be
informed about … debian11.
If you look at the logs when tomcat starts geoserver it should say
something.
As an example a common failure is in ability to read the file system
location where the GEOSERVER_DATA_DIR is located. Tomcat can be configured
to sandbox file system access or something. Perhaps that was the default
when you updated?
Please have a look at the logs and report back why geoserver failed to
start - what the first error or exception was. After the first error there
are usually more (if it has problems starting up it will also have problems
shutting down).
To get the logs I would have to make the service unavailable, restore the original VM, ~ 1Tb, get the logs, restore the modified (working) VM. this is not feasible.
To get the system up post the OS upgrade I initiated the following holds
apt-mark showhold
libtomcat9-java
openjdk-11-jre
tomcat9
tomcat9-admin
tomcat9-common
tomcat9-docs
tomcat9-examples
tomcat9-user
This in my opinion narrows the issue down to one of the held items.
OK, docs, examples may not be relavent, but I needed to get it working so I went for the full option