CMIP7: UKESM1-3-LL: Investigate missing grid_label from CV

Upon testing the model UKESM1-3-LL, zostoga_tavg-u-hm-sea (mip_convert_onm_ocean-global-mean-grid-T) gave the cmor.write error.

"
! Error: Your attribute grid_label is set to "g100m" which is invalid.
!
! Check your Controlled Vocabulary file
"

In order to produce this variable using this grid, an amendment to the CV file and any other associated requirements will need to be made.

Start with the reported cmor.write failure for UKESM1-3-LL and the variable zostoga_tavg-u-hm-sea, then review the Controlled Vocabulary file for the grid_label value g100m. Identify any associated requirements needed for this grid, and consider the issue done when the variable can be produced without the invalid-grid_label error.

Written by the indexing model from the issue text.