Update Nanum weight values

[emmamarichal]

In the metadata.pb files for Nanum Gothic and Nanum Myeongjo, the weights for bold and extrabold are set as 600 and 700.
They should be 700 and 800.

Metadata on google3 has 700 and 800, so we must have manually changed it when we first imported the fonts. I'll continue with the same values and in that case it should be safe to change the values on github.