Using MySQL ver. 5.5.55 in a production env
There are four reasons to upgrade the software:


Significant bug was removed
Security breach was removed
Hardware have
to be changed
Database structure was modified dramatically and
require some features not available in the older versions.


Updates performed just for software actuality are inappropriate in the production.

phpMyAdmin is well-known as unsecure software with lot of breaches so its location should be at least password-protected and restricted to the secure subnet(s).