Move from `solarradiation` to `netsolarradiation`

[veenstrajelmer]

The quantity netsolarradiation was implemented in FM in Deltares/Delft3D#38. This was merged on 14 August 2025, so first available in DIMRset 2.29.19 and will be included in the 2026.01 release.

Todo:

• keep downloading netsolarradiation/ssr
• await 2026.01 release
• update to this FM version at EDITO: https://gitlab.mercator-ocean.fr/pub/edito-infra/process-helm-charts/-/merge_requests/40
• consider renaming from surface_net_solar_radiation to surface_net_downward_shortwave_flux in dfmt.download_ERA5() >> won't do
• rename fm quantity from solarradiation to netsolarradiation in preprocess_merge_meteofiles_era5()
• remove 0.94 correction in xarray_helpers.convert_meteo_units()
• await add netsolarradiation HYDROLIB-core#997
• await release and update minimal hydrolib-core version (0.10.0)
• add test >> added but will only pass after hydrolib-core fix
• fix test_download_hycom #1288
• discuss with FZ about the timing, is 2026.01 already used for DCSM everywhere? >> after DCSM release in 2026, since for current DCSM release users officially have to use a FM kernel version that does not support netsolarradiation yet.
• update whatsnew