Introduce a configurable list of old extensions considered as flavors

To determine which flavor is installed on a wiki, we look at the installed extensions with the category "flavor".
Of course, old extensions does not have this category because it did not exist at the time.
So, we need a list of old extensions that we now consider as flavor.
The variable "extensions.oldflavors" is already used in FlavorManager (introduced by XWIKI-11741), but the variable is currently not present in xwiki.properties.