Update links to example research code where that code is old and unmaintained

[StevenMaude]

In #1572, it was noted that there is at least one link to research code that doesn't do things exactly as might be the OpenSAFELY best practice today (it uses run: jupyter:latest and not run: python:v2).

Links to code repositories are currently in:

• [x] docs/data-sources/apc.md
• [x] docs/data-sources/ecds.md
• [x] docs/data-sources/systmone.md
• [x] docs/data-sources/cpns.md
• [ ] docs/federation.md
• [ ] docs/protocol.md
• [ ] docs/reports/intro.md
• [x] docs/study-def-flowcharts.md
• [x] docs/study-def-tricks.md

If these are outdated, we should replace them with more current examples.