Migrate everything to `just` and remove Makefile(s)

[petros]

We have one last Makefile in backend-e2e. There's an opportunity here to migrate that to just since that's what we are using in the rest of the project. Once that's done, we can do away with the Makefiles, and the requirement to install build-essential as recommended in:

https://github.com/liveask/liveask/blob/008c42d7316697327691b6688bd105fac66b4156/CONTRIBUTION.md?plain=1#L15-L16