Minor changes

List: Parameterize generic T for item type with default type any (commit by KevinTCoughlin, pr #8465)

Patches

Calendar: Making calendar days aria-readonly: true so that Narrator doesn't read them as editable. (commit by khmakoto, pr #8447)
DocumentCard: Removing blank 1px line below preview images in compact layout. (commit by khmakoto, pr #8461)
Remove custom high contrast selectors and example utility CSS classes (commit by ecraig12345, pr #8467)
Split up color utility functions into separate files for perf (commit by phkuo, pr #8468)
GroupHeader: tighten componentWillReceiveProps parameter typing (commit by natalieethell, pr #8484)
update API file (commit by natalieethell, pr #8477)
update api file generated by api-extractor 7 (commit by natalieethell, pr #8459)