[8.0] Upgrade to league/oauth2-server 8.0

[matt-allan]

This PR updates the league/oauth2-server package to 8.0 and updates Passport to be compatible with it.

The changes are documented in the upgrade guide. The things I had to change for Passport are:

• Set the AccessToken client property when creating an access token. This isn't documented in the upgrade guide, but the server no longer calls setClient before calling getClient. To prevent a fatal error we need to set the client before returning the token from AccessTokenRepository::getNewToken.

• Set the Client isConfidential property when creating a client. The League server checks this property to determine if it should validate the client secret. This will always be true for now; I'm going to add support for public clients in a separate PR.

• Split the ClientRepository::getClientEntity method into two methods, getClientEntity and validateClient. The league server calls validateClient for all grants except for implicit grants and authorization code clients when the client does not have a secret.

• Pass a DateTimeImmutable instead of DateTime to AccessToken::setExpiryDateTime. This only affected a test.

This PR adds PKCE support (#837) but it's not really useful until Passport supports public clients. As mentioned above I'm working on a separate PR to support public clients.

In addition to running the test suite I manually tested this with all of the grant types and tested PCKE support.

[matt-allan]

I have support for public clients working in this branch but it will need to be rebased once this PR is merged.

[matt-allan]

@driesvints I made the requested changes.

Changes were made.

[jeanlucnguyen]

@matt-allan Did you have a chance to rebase the branch supporting public clients since this PR has been merged?

[matt-allan]

Yep, I opened a PR but closed it for now because I needed to clarify some behavior with the League oauth-server library.

I either need to make another PR to the League server or rework the Passport PR. I'm hoping to have some time for open source at the end of this week.