Removed all pre-2023 projects.

[pbellec]

Closes issue #279
It removes all projects from previous editions, and only retains projects from the current edition.
I've also renamed the folder of Justine's project from mytemplate to something a bit more explicit.

[FrancoisPgm]

There is still the Hinana_project which is from last year, and appears just as a copy of the template project, since she didin't put the info about her project in the index.md file but in a brain-hack-school text file. I suggest we remove it too.

[clarkenj]

Looks good, just on the right-hand side it says "THIS IS THE PROJECT GALLERY FROM 2020-21". I'm not sure which file this is in.

[pbellec]

Alright, I am going to close this for now. We can decide in 2025 how to deal with past editions, but for now we can keep the full gallery.