MySQL Dying After a Few Days
[Note] InnoDB: Initializing buffer pool, size = 128.0M
InnoDB: mmap(137363456 bytes) failed; errno 12


errno 12 is Out of memory - So probably you did not set any value for the innodb_buffer_pool_size and the default is used. Depending on your previous version the default might change on upgrade. Or you did have it configured but the upgrade rewrote your config? Or your new Ubuntu just needs more memory for itself.

Change the value, add more RAM, find some memory hog which might eat your RAM or enable swap (might slow everything down when it hits the swap).