Patch radiant-mlhub-landcovernet tutorial

[m-cappi]

Bug:

• Land Cover Net global dataset is no longer available

Fix:

• Use the Land Cover Net Africa dataset instead

Changes:

• Had to drop the use of pystac.extensions.label.LabelExtension in favor of accessing the labels as JSONs
• Made some content adjustments in order to reflect the current changes

[TomAugspurger]

Thanks @m-cappi!