Change identityprovider to assertingparty in Saml2RelyingPartyProperties

[marcusdacoregio]

Since this commit in Spring Security, some properties in the RelyingPartyRegistration were deprecated in favor of others aligned SAML 2.0 Spec Language.

Consider changing identityprovider in favor of assertingparty in Saml2RelyingPartyProperties to better align with the naming in Spring Security.

Related:

• RelyingPartyRegistration should use metadata spec language spring-security#8777

[mhalbritter]

I've added a note to the release notes: https://github.com/spring-projects/spring-boot/wiki/Spring-Boot-2.7.0-RC1-Release-Notes#deprecations-in-spring-boot-27