Refactor oauth2-resource-server deprecated calls

[paulvas]

This pull request refactors tests for both SpringOpaqueTokenIntrospector and SpringReactiveOpaqueTokenIntrospector to use their new builder APIs instead of direct constructors. It also updates tests to reflect changes in how client credentials encoding is handled, ensuring that special characters are now supported. Additionally, the tests for constructor argument validation have been updated for consistency with the new builder pattern.

Builder API migration:

• Replaced direct constructor calls with the new builder pattern (withIntrospectionUri, clientId, clientSecret, build) in all relevant test cases for both SpringOpaqueTokenIntrospector and SpringReactiveOpaqueTokenIntrospector. [1] [2] [3] [4]

Constructor validation updates:

• Updated tests for null/empty constructor arguments to use the builder API, and changed one test to validate null instead of empty string for clientId in the reactive introspector. [1] [2]

Client credentials encoding changes:

• Removed tests that expected failures when client credentials contained special characters, and updated them to expect successful authentication and correct attribute mapping, reflecting improved support for special characters in credentials. [1] [2] [3]