Add Spring Authorization Server project: OAuth2/OIDC provider, client and resource server

Three modules on Spring Boot 4.1.1 with Spring Authorization Server 7.1.1: the provider
itself, a relying party, and an API that trusts its tokens. Client registration, PKCE,
a custom consent page and token customisation, with profiles that make each failure
reproducible.

Every claim is backed by captured output in docs/output/as-*.txt, regenerated by
authorization-server/scripts/run-all.sh. Notable findings, verified against the jars:

  - OAuth2AuthorizationServerConfiguration.applyDefaultSecurity(HttpSecurity) was deleted
    in 7.0, and both configuration classes moved into spring-security-config
  - ClientSettings.requireProofKey flipped from false to true, on the authorization server
    (1.5.8 -> 7.1.1) and on the OAuth2 client (6.5.1 -> 7.1.1)
  - requireProofKey(false) does not make PKCE optional for a public client; the code
    verifier is that client's only authentication at the token endpoint
  - MediaTypeRequestMatcher(TEXT_HTML) matches Accept: */*, so the token endpoint answers
    API callers with 302 -> /login unless setIgnoredMediaTypes(ALL) is called

Also renames the repository to spring-auth-demo and cross-links the new chapter set from
the existing documentation.

docs/11-spring-security-7-changes.md

@@ -130,3 +130,19 @@ ankurm.com has a dedicated
 [gh-18926]: https://github.com/spring-projects/spring-security/issues/18926
 [gh-18634]: https://github.com/spring-projects/spring-security/pull/18634
 [gh-18113]: https://github.com/spring-projects/spring-security/issues/18113
+
+---
+
+Two more 7.x changes surfaced while building the authorization-server project, both
+verified by reading the jars rather than the release notes:
+
+- `ClientSettings.requireProofKey` flipped from `false` to `true` on **both** the
+  authorization server and the OAuth2 client —
+  [`authorization-server/03`](authorization-server/03-clients-and-pkce.md)
+- `OAuth2AuthorizationServerConfiguration.applyDefaultSecurity(HttpSecurity)` was deleted,
+  and its class moved into `spring-security-config` —
+  [`authorization-server/02`](authorization-server/02-minimum-provider.md)
+
+`FactorGrantedAuthority` now appears in every authority list, and `WWW-Authenticate` carries
+an RFC 9728 `resource_metadata` parameter —
+[`authorization-server/06`](authorization-server/06-resource-server.md).