Removal of osc:themes

[silvester-pari]

FYI, starting with v1.0.0-rc.7, the osc:themes property has been generally removed in favor of the more generic themes extension.

See the diff here: https://github.com/ESA-EarthCODE/open-science-catalog-validation/pull/14/files#diff

[TejasMorbagal]

@silvester-pari Thanks for the issue. Working on it.