era5: update to new CDS infrastructure

[fneum]

closes #361

This PR is backwards compatible; old CDS infrastructure can still be used.

Other things needed to update:

• follow instructions https://forum.ecmwf.int/t/the-new-climate-data-store-beta-cds-beta-is-now-live/3315
• retrieve new ~/.cdsapirc
• update to cdsapi>=0.7.0

Main changes:

• The time variable was renamed in CDS from "time" to "valid_time", which is renamed in _rename_and_clean_coords()
• A useless "number" coordinate was added, which is removed in _rename_and_clean_coords().
• There was some weirdness in the "expver" coordinate when ERA5 and ERA5T data were mixed (coordinate but not a dimension). Also, it seems that the "expver" coordinate is now always created and is a 4-digit string rather than an integer. If somebody has a leaner suggestion here, I am all ears. It was quite tricky as non-xarray expert.

If we want to use new CDS infrastructure in CI, the secrets need to be updated. Perhaps we shouldn't do this immediately.

Furthermore, I am not sure how to remove the plentiful INFO logs.

[FabianHofmann]

Don't understand it fully, but looks good

[euronion]

• Should restrict to new CDSAPI package version
• Should add migration note in the release notes
• Should make a new release

I think migrating ASAP makes sense, since the old one will be shutdown in less than a month.