Remove limitation for SAML encryption in FIPS mode (#48948)

Our documentation regarding FIPS 140 claimed that when using SAML
in a JVM that is configured in FIPS approved only mode, one could
not use encrypted assertions. This stemmed from a wrong
understanding regarding the compliance of RSA-OAEP which is used
as the key wrapping algorithm for encrypting the key with which the
SAML Assertion is encrypted.

However, as stated for instance in
https://downloads.bouncycastle.org/fips-java/BC-FJA-SecurityPolicy-1.0.0.pdf
RSA-OAEP is approved for key transport, so this limitation is not
effective.

This change removes the limitation from our FIPS 140 related
documentation.

x-pack/docs/en/security/fips-140-compliance.asciidoc

@@ -121,5 +121,3 @@ features are not available while running in fips mode. The list is as follows:
   can be later used in the FIPS 140-2 enabled JVM.
 * The SQL CLI client cannot run in a FIPS 140-2 enabled JVM while using
   TLS for transport security or PKI for client authentication.
-* The SAML Realm cannot decrypt and consume encrypted Assertions or encrypted
-  attributes in Attribute Statements from the SAML IdP.