Replace or remove samltest.id references

Problem/Motivation

The documentation for setting up a test IdP describes using samltest.id which appears to no longer be a valid service as of at least February 2024.

We should either document an alternative free IdP setup or remove references to this as it is currently confusing.

Steps to reproduce

Visit https://samltest.id

Proposed resolution

Remove section "SAMLtest.id Identity Provider for testing" unless an alternative is discovered or the service returns.