Fix broken links

Repeatedly, need to fix (meanwhile) broken links.

From https://codebase.helmholtz.cloud/hifis/overall/hifis.net/-/jobs/1672755#L181 :

• At public/blog/2019/12/20/Welcome-Post.html:857:
  External link https://dlr.de/sc failed: Not Found (status code 404)
• At public/news/2020/03/13/jsc-hpc-support-for-covid-19-research.html:753:
  External link https://www.fz-juelich.de/SharedDocs/Meldungen/IAS/JSC/EN/2020/offer-covid-19.html failed (status code 404)
• At public/news/2020/11/20/helmholtz-aai-policies.html:804:
  External link https://aarc-community.org/policies/ failed: Not Found (status code 404)
• At public/news/2021/10/22/helmholtz-aai-full-presentation-in-dfn-meeting.html:810:
  External link https://codebase.helmholtz.cloud/hifis/overall/kpi/kpi-plots-ci/-/jobs/artifacts/master/file/plots/202102_Backbone/plot.png?job=plot_general failed (status code 404)
• At public/news/2021/11/24/Software-Spotlights.html:826:
  External link https://os.helmholtz.de/en/open-science-in-the-helmholtz-association/open-research-software/ failed (status code 404)
• At public/news/2022/12/12/evaluation-update.html:858:
  External link https://syncnshare.desy.de/index.php/f/467281196 failed: Unauthorized (status code 401)
• At public/news/2024/02/02/hfmi.html:850:
  External link https://events.hifis.net/e/matchmakingHFMI failed: Bad Request (status code 400)
• At public/publications.html:800:
  External link https://betterscientificsoftware.github.io/swe-cse-bof/2019-11-sc19-bof/04-haupt-helmholtz.pdf failed (status code 404)
• At public/roadmap/2024/05/01/new-HIFIS-coordinator.html:2:
  External link https://hifis.net/news/2024/06/06/neumann failed (status code 404)
• At public/spotlights/cff.html:740:
  External link https://ev.campussource.de/publikationen/csa2022/ failed: Not Found (status code 404)
• At public/spotlights/golem.html:778:
  External link https://helmholtz.software/software/golem-a-moose-based-application failed (status code 404)
• At public/spotlights/lynx.html:776:
  External link https://helmholtz.software/software/lynx-modelling-lithosperic-dynamics failed (status code 404)
• At public/spotlights/palladio.html:865:
  External link https://www.palladio-simulator.com/the_book/ failed (status code 404)
• At public/timeline/index.html:1519:
  External link https://hifis.net/news/2024/06/06/neumann failed (status code 404)
• https://hifis.net/tutorial/2019/11/22/How-to-Create-a-new-Blog-Post.html#References
  Reference 8 broken