Resolve #3624202 "Clean dev release 2.x"

https://www.drupal.org/project/toolbar/issues/3624202

Initial round of changes to create a 2.x-dev release and prepare for a stable version:

• Added composer.json with name drupal/toolbar-toolbar to go through composer jobs in pipelines.
• Added .gitlab-ci.yml with security jobs.
• Removed VERSION information in YAML files such as libraries, info, etc...
• Added support for ^11.5 to comply with the exact version Toolbar is deprecated in core (11.5.0).
• Fixed jobs cspell, nightwatch, phpcs, phpstan and phpunit.
• Fixed remaining fixed path references to core/modules/toolbar .

This merge request is based on pieces from MR !1 (merged) and MR !2 (closed).