Deprecations for next major php version

Problem/Motivation

While running tests of the next major PHP version for OpenID Connect, we started seeing the following deprecation messages coming from ExternalAuth:

Deprecated: Drupal\externalauth\Authmap::delete(): Implicitly marking parameter $provider as nullable is deprecated, the explicit nullable type must be used instead in /builds/project/openid_connect/web/modules/contrib/externalauth/src/Authmap.php on line 123

Deprecated: Drupal\externalauth\AuthmapInterface::delete(): Implicitly marking parameter $provider as nullable is deprecated, the explicit nullable type must be used instead in /builds/project/openid_connect/web/modules/contrib/externalauth/src/AuthmapInterface.php on line 90

Those messages are displaying here: https://git.drupalcode.org/project/openid_connect/-/jobs/3848290#L188

Proposed resolution

Change the method definition from:

public function delete(int $uid, string $provider = NULL) {

to

public function delete(int $uid, ?string $provider = NULL) {