Werk #15671: SAML: use RSA-SHA256 to sign authentication requests

Checkmk would sign its authentication requests with RSA-SHA512. However, some identity providers (e.g. some versions of Microsoft ADFS) do not support any signature algorithms beyond SHA256. As a result, the authentication requests would be rejected with an error message similar to

"Error details: MSIS7093: The message is not signed with expected signature algorithm. Message is signed with signature algorithm http://www.w3.org/2001/04/xmldsig-more#rsa-sha512. Expected signature algorithm http://www.w3.org/2001/04/xmldsig-more#rsa-sha256."

For this reason, Checkmk now uses RSA-SHA256 to sign its authentication requests.

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