Update DigiCert SSP that they are shutting down

[idmken]

Update - https://github.com/GSA/idmanagement.gov/blob/federalist-pages/_buy/trust-services.md

DigiCert is transitioning out of the Shared Service Provider program and Federal PKI.

This page should include a note under the government identity section that - DigiCert is transitioning out of the Shared Service Provider program and no longer offer services to new customers and transitioning existing customers to a new SSP. DigiCert should be removed from the table.