Replace ERA-Interim with ERA5 where possible

[rbeucher]

ERA-Interim has been discontinued and is no longer updated as of 31 August 2019. ERA5 is now the recommended dataset.
ERA-Interim is also in Tier3 and is difficult to make available.

[flicj191]

Had a look at this with the native6 facets and native6: /g/data/rt52/era5 but it wasn't loading the data, which is because the files are one per month for monthly data so one time point and raises error: https://github.com/ESMValGroup/ESMValCore/blob/4c36a0c7be2a48b32e1394e6e3212c9f7913a2f7/esmvalcore/cmor/_fixes/native6/era5.py#L26

Otherwise looks like it would work if it knows the monthly frequency