Remove further_info_url from MIP Convert code

Mip Convert requires there to be a `further_info_url` in the global attributes, but this is (post-CMIP6) no longer useful as the system is deprecated.

This will likely require:
* removal of code from MIP Convert
* removal of requirement from gcmodeldev-cmor-tables (if necessary)

Attributes like this can still be added in the global attributes section, but should not be required.

Start by locating the MIP Convert code that validates the global `further_info_url` attribute. Then inspect gcmodeldev-cmor-tables to determine whether the requirement is defined there as well. Done means the attribute is no longer required, while users can still provide it in global attributes.

Written by the indexing model from the issue text.