Rename 'identityprovider' property to 'asserting-party'

Rename spring.security.saml2.relyingparty.registration.*.identity-provider.*
to spring.security.saml2.relyingparty.registration.*.asserting-party.*

The old property names are still supported, but will lead to a warning
in the logs.

Closes gh-30642

spring-boot-tests/spring-boot-smoke-tests/spring-boot-smoke-test-saml2-service-provider/src/main/resources/application.yml

@@ -8,10 +8,10 @@ spring:
               credentials:
               - private-key-location: "classpath:saml/privatekey.txt"
                 certificate-location: "classpath:saml/certificate.txt"
-            identityprovider:
+            asserting-party:
               verification:
                 credentials:
-                - certificate-location: "classpath:saml/certificate.txt"
+                  - certificate-location: "classpath:saml/certificate.txt"
               entity-id: simplesaml
               singlesignon:
                 url: https://simplesaml-for-spring-saml/SSOService.php
@@ -21,10 +21,10 @@ spring:
               credentials:
               - private-key-location: "classpath:saml/privatekey.txt"
                 certificate-location: "classpath:saml/certificate.txt"
-            identityprovider:
+            asserting-party:
               verification:
                 credentials:
-                - certificate-location: "classpath:saml/certificate.txt"
+                  - certificate-location: "classpath:saml/certificate.txt"
               entity-id: okta-id-1234
               singlesignon:
                 url:

spring-boot-tests/spring-boot-smoke-tests/spring-boot-smoke-test-saml2-service-provider/src/test/java/smoketest/saml2/serviceprovider/SampleSaml2RelyingPartyApplicationTests.java

@@ -46,7 +46,7 @@ class SampleSaml2RelyingPartyApplicationTests {
 	}
 
 	@Test
-	void loginShouldHaveAllIdentityProvidersToChooseFrom() {
+	void loginShouldHaveAllAssertingPartiesToChooseFrom() {
 		ResponseEntity<String> entity = this.restTemplate.getForEntity("/login", String.class);
 		assertThat(entity.getStatusCode()).isEqualTo(HttpStatus.OK);
 		assertThat(entity.getBody()).contains("/saml2/authenticate/simplesamlphp");