Fix deprecations so that PHPStan CI jobs pass

It is a little tricky to fix this deprecation. The message I get (from GitLab CI or from running phpstan locally) is

Line src/EventSubscriber/OpenIDConnectAutoLogin.php
------ -----------------------------------------------------------------------------------------
211 Call to deprecated method get() of class Symfony\Component\HttpFoundation\Request:
since Symfony 7.4, use properties `->attributes`, `query` or `request` directly instead
🪪 method.deprecated

So the first step is to figure out whether to use attributes or query or request. The line in question is

    $route_name = $request->get(RouteObjectInterface::ROUTE_NAME);

and RouteObjectInterface is in the Drupal\Core\Routing namespace. So check that namespace for usage hints:

$ grep -r ROUTE_NAME core/lib/Drupal/Core/Routing
core/lib/Drupal/Core/Routing/RouteObjectInterface.php:  const ROUTE_NAME = '_route';
core/lib/Drupal/Core/Routing/UrlMatcher.php:    $attributes[RouteObjectInterface::ROUTE_NAME] = $name;
core/lib/Drupal/Core/Routing/RouteMatch.php:        $request->attributes->get(RouteObjectInterface::ROUTE_NAME),

It sure looks like an attribute!

My first thought was to replace the offending line with

    $route_name = $request->attributes->get(RouteObjectInterface::ROUTE_NAME);

That works: PHPStan is happy (except for the next-minor and next-major CI jobs).

Then I had another idea: use the RouteMatch class instead of the low-level Symfony objects. This works, and it makes the line a little shorter:

    $route_name = RouteMatch::createFromRequest($request)->getRouteName();

(Also replace the use statement.)

The first change gets the phpstan CI job to pass.

It is not too hard to get the phpstan (next minor) CI job to pass. The messages are

Line openid_connect.module
------ ----------------------------------------------------------------------------
165 Call to deprecated function user_load_by_mail():
in drupal:11.4.0 and is removed from drupal:13.0.0.
Use \Drupal::entityTypeManager()->getStorage('user')->loadByProperties()
instead.
🪪 function.deprecated
168 Call to deprecated function user_load_by_name():
in drupal:11.4.0 and is removed from drupal:13.0.0.
Use \Drupal::entityTypeManager()->getStorage('user')->loadByProperties()
instead.
🪪 function.deprecated

This time, there is a change record telling us what to do: user_load_by_mail() and user_load_by_name() are deprecated.

The phpstan (next major) CI job is harder. There are a bunch of messages like this (all in the same file):

Line tests/src/Unit/OpenIDConnectTest.php
------ ----------------------------------------------------------------------
192 Call to deprecated method any() of class PHPUnit\Framework\TestCase:
https://github.com/sebastianbergmann/phpunit/issues/6461
🪪 method.deprecated

These deprecations were only recently fixed in Drupal core: see #3561671: [meta] Refactor tests to use stubs instead of mocks where mocks do not configure expectations. I suggest we first get to a stable release of this module on the 3.x branch, and then worry about fixing PHPStan for Drupal 12.