Resolve PHP deprecations

Problem/Motivation

There is at least one instance of a recent php change causing deprecation notices to be emitted. This issue should fix them.

Deprecated function: Drupal\simplesamlphp_auth\Service\SimplesamlphpAuthManager::__construct(): Implicitly marking parameter $instance as nullable is deprecated, the explicit nullable type must be used instead in include() (line 576 of /var/www/html/vendor/composer/ClassLoader.php).

Deprecated function: Drupal\simplesamlphp_auth\Service\SimplesamlphpAuthManager::__construct(): Implicitly marking parameter $config as nullable is deprecated, the explicit nullable type must be used instead in include() (line 576 of /var/www/html/vendor/composer/ClassLoader.php).

Steps to reproduce

1. Enable error logging.
2. Flush the cache
3. Load a page
4. See the notice

Proposed resolution

Resolve all deprecation notices.

Remaining tasks

None

User interface changes

None

API changes

None

Data model changes

None