correct attributions without years

[lucasrodes]

As per our documentation, the attribution to the data provider should always specify the year of publication of the data (except for very rare occasions).

Some datasets (and indicators) ' metadata lacks the year. We should fix these.

"UNICEF; World Health Organization; World Bank"

• https://github.com/owid/etl/blob/master/etl/steps/data/grapher/who/2024-01-03/gho.meta.override.yml#L35
• https://github.com/owid/etl/blob/master/etl/steps/data/grapher/worldbank_wdi/2023-05-29/wdi.meta.yml#L158
• https://github.com/owid/etl/blob/master/etl/steps/data/grapher/worldbank_wdi/2024-05-20/wdi.meta.yml#L158
• https://github.com/owid/etl/blob/master/etl/steps/data/grapher/worldbank_wdi/2024-05-20/wdi.meta.yml#L250

[pabloarosado]

We discussed this kind of issue in the past. Maybe we could consider doing something along the lines of #2220
Not necessarily to ensure all guidelines are fulfilled, but at least some of them that could be easy to spot programmatically.

Mojmir proposed that this could be a separate linter, e.g. make check (but for metadata instead of code), instead of being part of ETL create_datasets (which could add execution time and other issues). Maybe OWIDBOT could print a summary of such issues.