Fix PHP 8.1 support

Problem/Motivation

 [success] Successfully enabled: migrate_generator, migrate, migrate_plus, migrate_source_csv, migrate_tools
docker-compose exec -T --user 1000:1000 php  drush migrate_generator:generate_migrations /var/www/html/content --update
 [warning] Undefined array key 3 Scanner.php:110
 [warning] Undefined array key 1 Scanner.php:176
 [warning] Undefined array key 1 Scanner.php:176
 [warning] Undefined array key 1 Scanner.php:176
 [warning] Undefined array key 1 Scanner.php:176
 [warning] Undefined array key 1 Scanner.php:176
 [warning] Undefined array key 1 Scanner.php:176
 [warning] Undefined array key 1 Scanner.php:176
 [warning] Undefined array key 1 Scanner.php:176
 [notice] Migration mgg__taxonomy_term__category was created
 [notice] Migration mgg__user was created
 [notice] Generation of migrations was completed. You can now run them using next command:
drush migrate:import --all --group=mgg
docker-compose exec -T --user 1000:1000 php  drush migrate:import --all --group=mgg

 1/4 [=======>--------------------]  25%
 4/4 [============================] 100% [notice] Processed 4 items (4 created, 0 updated, 0 failed, 0 ignored) - done with 'mgg__taxonomy_term__category'

 1/1 [============================] 100% [notice] Processed 4 items (4 created, 0 updated, 0 failed, 0 ignored) - done with 'mgg__taxonomy_term__category'
 [notice] Processed 1 item (1 created, 0 updated, 0 failed, 0 ignored) - done with 'mgg__user'

Steps to reproduce

Use with https://github.com/skilld-labs/skilld-docker-container/pull/412

Proposed resolution

fix erors

Remaining tasks

User interface changes

API changes

Data model changes