go.mod: update gosaml2 dep to include SAML metadata endpoint fix
Created by: beyang
Relevant upstream diff: https://github.com/sourcegraph/gosaml2/commit/5cfddeb48b17ee77306e3833a49abd886c82adf9
This fixes a panic where the /.auth/saml/metadata
HTTP endpoint would panic and return an error if no SAML encryption key pair was configured (encryption is optional). This broke our Microsoft ADFS SAML instructions.