Fix CI: Pin versions, tidy up upload-artifact calls

[MakisH]

Description

• Pins the pandoc/latex version to 2 -> Fixes the undefined ref issue, triggered in version 3. Not sure if it is worth investing more effort here.
• Bumps the runner version to 22.04 (could even be latest), as the 20.04 should anyway be EOL.
• Fixes an image URL that returned 404.
• Removes duplicate calls to upload-artifacts and names the remaining two (slides and texts)

This can be merged as-is, without squashing.

Checklist

• I made sure that the markdown files are formatted properly. -> N/A
• I made sure that all hyperlinks are working.

• I added the new content to the README.md inside the topics subdirectory, e.g., 01_version_control/README.md. -> N/A
• I added the new content to the timetable.md in the root of this repository. -> N/A

Please check that the uploaded artifacts are as intended.

[uekerman]

Thanks!