Update README with CVE-2025-66475 reference

Author: pitbulk

README.md

@@ -12,6 +12,8 @@ Add SAML support to your PHP software using this library.
 Warning
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+Version 2.21.1 updates xmlseclibs to 3.1.4 due [CVE-2025-66475](https://github.com/advisories/GHSA-c4cc-x928-vjw9)
+
 Version 2.18.0 introduces the 'rejectUnsolicitedResponsesWithInResponseTo' setting parameter, by default disabled, that will allow invalidate unsolicited SAMLResponse. This version as well will reject SAMLResponse if requestId was provided to the validator but the SAMLResponse does not contain a InResponseTo attribute.  And an additional setting parameter 'destinationStrictlyMatches', by default disabled, that will force that the Destination URL should strictly match to the address that process the SAMLResponse.
 
 Version 2.17.1 updates xmlseclibs to 3.0.4 (CVE-2019-3465), but php-saml was not directly affected since it implements additional checks that prevent to exploit that vulnerability.