Use Spring OAuth2 Resource Server over custom implementation

As part of the 5.2.0 release of Spring Security, we can use the OpaqueTokenIntrospector feature of Spring Security OAuth2 Resource Server to replace a number of security-related features we had previously created with out-of-the-box Spring ones.

This requires an upgrade to spring-security and spring-boot-starter (as it pulls in underlying Spring Web changes) to use the new functionality.

Closes #53 (closed).

added enhancement label

mentioned in commit 247e31e6

merged