Drupal 11 compatibility: ConfigFormBase fix, config schema correction, and code modernization

Problem/Motivation

The message_banner module is incompatible with Drupal 11 due to several issues:

1. ConfigFormBase constructor change - In Drupal 11, ConfigFormBase::__construct() requires a second parameter (TypedConfigManagerInterface). The current code calls parent::__construct($config_factory) which causes ArgumentCountError: Too few arguments to function Drupal\Core\Form\ConfigFormBase::__construct().
2. Incorrect config schema type - The config schema uses type: config_entity but this is simple configuration, not a config entity. Should be type: config_object.
3. Deprecated AccountInterface constants - The install hook uses AccountInterface::AUTHENTICATED_ROLE and AccountInterface::ANONYMOUS_ROLE which are deprecated.
4. Missing PHP 8.1+ features - Properties lack type declarations, methods lack return types.

Steps to reproduce

1. Install Drupal 11
2. Install message_banner module
3. Navigate to /admin/config/user-interface/message-banner
4. Error: ArgumentCountError: Too few arguments to function Drupal\Core\Form\ConfigFormBase::__construct()

Proposed resolution

1. Use parent::create() pattern for D11 ConfigFormBase compatibility (per https://www.drupal.org/node/3404140)
2. Fix config schema: config_entity → config_object
3. Replace deprecated AccountInterface constants with string literals
4. Add typed properties and return types
5. Update core_version_requirement to ^10.3 || ^11
6. Update composer.json with PHP 8.1+ requirement

Remaining tasks

- Review and test
- Merge

User interface changes

none

API changes

none

Data model changes

None (config schema type correction is a fix, not a change to stored data)