The base theme property in theme .info.yml files is now required

In the past, Drupal used to default to the stable theme as the base theme. Explicitly opting out by specifying base theme: false was (and still is) possible.

However, defaulting to base theme: stable prevents automatic updates to the next major version of Drupal, since each major version may have a different version of "the stable theme":

• for Drupal 8: stable
• for Drupal 9: stable9
• for Drupal 10: stable10
• et cetera

It is impossible to reliably determine which of these should be selected by default, hence we now require the base theme to be explicitly specified.

Before

name: Foobar
type: theme
description: 'A foo bar-ish theme with much foo and excellent bar.'

After

name: Foobar
type: theme
description: 'A foo bar-ish theme with much foo and excellent bar.'
base theme: stable