Make aai-resources agnostic of the embedded server (remove jetty-specific code)

Make aai-resources agnostic of the embedded server (remove jetty-specific code)

- small adjustments to authentication
- make users configurable via aai.basic-auth.users[] in application.properties
- remove Keycloak integration [0]

[0] for the following reasons:
- integration test (MultiTenancyTest) already not working
- dependency is ancient (uses 11, 26 is available as of writing)
- keycloak autoconfiguration is in conflict with spring security
- keycloak-specific starters have been deprecated and users are advised to
  use the out-of-the-box spring OIDC integration [1]
- there is no (and likely never will be a) starter that is compatible with spring-boot 3

[1] https://www.keycloak.org/2023/03/adapter-deprecation-update

Issue-ID: AAI-4100
Change-Id: I8440f44106f9ba2cbd4addfa88efb63329365977
Signed-off-by: Fiete Ostkamp <Fiete.Ostkamp@telekom.de>