The 'core' key is removed from Views configuration

In Drupal 8 views configuration no longer stores the version of Drupal the view has been created with. If your module, theme, or profile has exported views in its config/install or config/optional directories then this will be need to be removed.

For example:

id: frontpage
label: Frontpage
module: node
description: 'All content promoted to the front page.'
tag: default
base_table: node_field_data
base_field: nid
core: 8.x
display:
# Lots
# more
# config...

becomes

id: frontpage
label: Frontpage
module: node
description: 'All content promoted to the front page.'
tag: default
base_table: node_field_data
base_field: nid
display:
# Lots
# more
# config...

This key will be removed from active configuration on update and even if a module does have exported configuration with the key it will be removed on module install.