NIFI-16301 Fix deprecated Spring Security SAML 2 Principal references

[exceptionfactory]

Summary

NIFI-16301 Replaces deprecated usage of Spring Security SAML 2 Principal methods with equivalent behavior using newer SAML 2 Assertion classes. Changes include additional tests for the updated assertion converter behavior.

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[pvillard31]

LGTM, thanks for addressing these deprecations @exceptionfactory