Update UM to vn13.1 for compatibility with CABLE

[MartinDix]

CABLE development is using UM vn13.1

[MartinDix]

vn13.0 to vn13.1 differences
https://code.metoffice.gov.uk/trac/um/changeset?reponame=&new=114076%40main%2Ftrunk%2Fsrc&old=111272%40main%2Ftrunk%2Fsrc

Large number of trivial differences.

[MartinDix]

After the move to new repository, rebased the vn13.0_nuopc branch onto the tagged UKMO_vn13.1 to create a vn13.1_nuopc branch.