Change last_updated_date YAML tag for data files

[ekrimmel]

For YAML files in the data/resources directory, we have a tag last_updated_date. This should be changed to last_modified_at so that the language is the same as for YAML frontmatter on pages.

Notes

• We need to update this tag in all of the existing files where it is currently used.

[adamancer]

Before I touch this, I could use a little more clarity on this field for data files. Is last_updated_date/last_modified_at the last time the resource was updated or the last time the KH-specific metadata was modified? Probably the former? Is there any reason to track the latter?