YAML format in default.services.yml may need updating

Symfony's YAML parser no longer supports a syntax that was used in default.services.yml prior to Drupal 9.1.0.

If you installed Drupal core prior to version 9.1.0 and have copied default.services.yml, you may need to update the syntax to be compatible with the latest versions of Symfony. This is an optional change in Drupal 8 or 9, but will be required in Drupal 10.

Before:

factory.keyvalue:
  {}
factory.keyvalue.expirable:
  {}

After:

factory.keyvalue: {}
factory.keyvalue.expirable: {}