New major version for Drupal >= 11.3.x

Problem/Motivation

The 2.x branch currently supports all Drupal versions >= 10.2 - this leads to problems with fixing code quality issues and updating tests for new PHPUnit version(s) that should be valid for both Drupal 10 and 11 (especially >= 11.3.x).
So this issue should be used to create a new major version for Drupal >= 11.3 while limiting the 2.x version to Drupal < 11.3.

Proposed resolution

• Create a new 3.x branch that only supports Drupal >= 11.3
• Limit existing 2.x branch to only support Drupal >= 10.3 and Drupal < 11.3
• Fix code quality/style issues für both module versions (see also #3502724: Fix code quality issues reported by PHPStan)
• Create new releases for both version branches

Remaining tasks

• Use different branches für Drupal < 11.3 and Drupal >= 11.3
• Fix code quality/style issues

User interface changes

n/a

API changes

n/a

Data model changes

n/a