Remove compiler warnings for spring-security-oauth2-resource-server

[rwinch]

See gh-18405 for more details.

[paulvas]

@rwinch, I am working on fixing deprecated calls here and ran into a couple issues:

1. SpringReactiveOpaqueTokenIntrospector constructor with arguments is deprecated, switching to use builder...
   Old constructor has assertion that clientId is not empty:

	@Deprecated(since = "6.5", forRemoval = true)
	public SpringReactiveOpaqueTokenIntrospector(String introspectionUri, String clientId, String clientSecret) {
		Assert.hasText(introspectionUri, "introspectionUri cannot be empty");
		Assert.hasText(clientId, "clientId cannot be empty");
		Assert.notNull(clientSecret, "clientSecret cannot be null");
		this.introspectionUri = URI.create(introspectionUri);
		this.webClient = WebClient.builder().defaultHeaders((h) -> h.setBasicAuth(clientId, clientSecret)).build();
	}

Builder method for clientId has assertion that clientId is not null.

public Builder clientId(String clientId) {
			Assert.notNull(clientId, "clientId cannot be null");
			this.clientId = URLEncoder.encode(clientId, StandardCharsets.UTF_8);
			return this;
		}

Now we have a test in SpringReactiveOpaqueTokenIntrospectorTests.constructorWhenClientIdIsEmptyThenIllegalArgumentException which starts failing once we switch to builder.

Should I update SpringReactiveOpaqueTokenIntrospector.clientId builder to check it's not empty, or update test to use null clientId?

SpringOpaqueTokenIntrospector is checking for nulls in both constructor and clientId builder method. No issue there.

	@Deprecated(since = "6.5", forRemoval = true)
	public SpringOpaqueTokenIntrospector(String introspectionUri, String clientId, String clientSecret) {
		Assert.notNull(introspectionUri, "introspectionUri cannot be null");
		Assert.notNull(clientId, "clientId cannot be null");
		Assert.notNull(clientSecret, "clientSecret cannot be null");
		this.requestEntityConverter = this.defaultRequestEntityConverter(URI.create(introspectionUri));
		RestTemplate restTemplate = new RestTemplate();
		restTemplate.getInterceptors().add(new BasicAuthenticationInterceptor(clientId, clientSecret));
		this.restOperations = restTemplate;
	}
...
	public Builder clientId(String clientId) {
		Assert.notNull(clientId, "clientId cannot be null");
		this.clientId = URLEncoder.encode(clientId, StandardCharsets.UTF_8);
		return this;
	}

2. Test method in SpringReactiveOpaqueTokenIntrospectorTests introspectWithoutEncodeClientCredentialsThenExceptionIsThrown is testing that there is an exception, when there are special characters in clientId / clientSecret. Builder methods URLEncode those values, so this test doesn't work now. Since it's testing deprecated constructor, should I remove it?

Same issue is in SpringOpaqueTokenIntrospectorTests introspectWithoutEncodeClientCredentialsThenExceptionIsThrown.