rename <name>-element in web-fragment.xml of mycore-base module

The name of the web-fragment of mycore-base should be renamed to

from mycore to mycore_base

to comply with the naming conventions of the other MyCoRe modules.

References in web-fragment.xml files of other modules need to be updated.

This is a breaking change and therefore only done in main branch

Application developers should look into their modules and update accordingly !!!