hyrax/Gemfile: Bump omniauth-saml to >= 2.1.3

omniauth-saml has already been updated to 2.1.3 by the previous commit.
This commit merely bumps the minimum version to 2.1.3 to prevent
CVE-2025-25293 under any circumstances regardless of the contents of
hyrax/Gemfile.lock.

Signed-off-by: Pascal Ernster <pascal.ernster@rub.de>