Upgrade Drupal 9 to 10 or 11

Basic stuff and Sources for now. To be continued...

Lenient
Code
# Install Composer Lenient for being able to place non-compatible packages.
# @see https://github.com/mglaman/composer-drupal-lenient
composer require mglaman/composer-drupal-lenient
# To allow a package to have a lenient Drupal core version constraint, you must add it to: extra.drupal-lenient.allowed-list
composer config --merge --json extra.drupal-lenient.allowed-list '["drupal/token"]'
# Now you can require the module in question
composer require drupal/token:1.10.0
Rector
Code
# Make sure you are at the project's root. Then, get and copy rector.php
composer require --dev palantirnet/drupal-rector
cp vendor/palantirnet/drupal-rector/rector.php .
# Run check, in this example for a particular module. Remove dry run when ready. 
vendor/bin/rector process web/modules/contrib/token --dry-run