'Messages' region renamed to 'Highlighted' for all themes

The 'Messages' region was included in the default regions that are loaded for every theme. The 'Messages' region has been renamed to 'Highlighted' as it is more descriptive of the visual styling of the region in the theme instead of describing the content that is expected in the region.

If you previously placed blocks in the 'Messages' region, you will have to re-place them in the 'Highlighted' region. If your custom or contrib theme overrides the default theme regions then you do not need to update the block placement.