Update assertingparty property usage in YAML snippets

Spring Boot 2.7 renamed spring.security.saml2.relyingparty.registration.*.identityprovider.*
to spring.security.saml2.relyingparty.registration.*.assertingparty.*.

Closes gh-12810.

Signed-off-by: Danilo Piazzalunga <danilopiazza@gmail.com>

docs/modules/ROOT/pages/servlet/saml2/login/authentication-requests.adoc

@@ -67,7 +67,7 @@ spring:
     saml2:
       relyingparty:
         okta:
-          identityprovider:
+          assertingparty:
             entity-id: ...
             singlesignon.sign-request: false
 ----

docs/modules/ROOT/pages/servlet/saml2/login/overview.adoc

@@ -125,7 +125,7 @@ spring:
       relyingparty:
         registration:
           adfs:
-            identityprovider:
+            assertingparty:
               entity-id: https://idp.example.com/issuer
               verification.credentials:
                 - certificate-location: "classpath:idp.crt"
@@ -839,11 +839,11 @@ spring:
           signing.credentials: &relying-party-credentials
             - private-key-location: classpath:rp.key
               certificate-location: classpath:rp.crt
-          identityprovider:
+          assertingparty:
             entity-id: ...
         azure:
           signing.credentials: *relying-party-credentials
-          identityprovider:
+          assertingparty:
             entity-id: ...
 ----