Fix yml and YML to YAML in vale and docs

What does this MR do?

As noticed when reviewing another MR, it seems our vale rule to check correct the usage of YAML (not yml, YML, or yaml), had a slight bug. The vale rule was searching only for yaml variants (any case), but was not searching for yml variants.

Markdownlint already searches for yaml variants, so we should set vale to do the work of looking for yml variants instead. By updating vale with the correct term, it found ~20 cases that were missed. This MR fixes those as well.

Related issues

Resolves #363774 (comment 965251720)

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