Make Stepup-saml-bundle with Symfony 8

[johanib]

See #134

Stepup-saml-bundle needs work to be be compatible with Symfony 8.

The get() function on the request is removed.

https://github.com/OpenConext/Stepup-saml-bundle/blob/main/src/SAML2/AuthnRequestFactory.php#L40
https://github.com/OpenConext/Stepup-saml-bundle/blob/main/src/SAML2/AuthnRequestFactory.php#L52

Also fix other errors / deprecations if applicable.