v2.20.0 : Add Symfony 8 support and drop support for not maintained versions

What's Changed

Refactor Request::get() usages by @dmaicher in https://github.com/knpuniversity/oauth2-client-bundle/pull/463
Update README for authenticator registration steps by @necobm in https://github.com/knpuniversity/oauth2-client-bundle/pull/467
Migrate XML config to PHP by @dmaicher in https://github.com/knpuniversity/oauth2-client-bundle/pull/464
Drop support for unmaintained Symfony versions by @dmaicher in https://github.com/knpuniversity/oauth2-client-bundle/pull/465
Add support for Symfony 8 by @dmaicher in https://github.com/knpuniversity/oauth2-client-bundle/pull/466

@dmaicher made their first contribution in https://github.com/knpuniversity/oauth2-client-bundle/pull/463
@necobm made their first contribution in https://github.com/knpuniversity/oauth2-client-bundle/pull/467

Full Changelog: https://github.com/knpuniversity/oauth2-client-bundle/compare/v2.19.0...v2.20.0