Upstreams for 3 plugins are gone in main branch

[Neko-Box-Coder]

When performing any plugin actions, the following error will be shown:

Failed to decode repository data https://raw.githubusercontent.com/cadnza/mxc/main/repo.json :
 json: cannot unmarshal number into Go value of type config.PluginPackages
Failed to decode repository data https://notabug.org/dustdfg/micro-mdtree/raw/master/repo.json :
 invalid character '<' looking for beginning of value
Failed to decode repository data https://notabug.org/dustdfg/micro-calc/raw/master/repo.json :
 invalid character '<' looking for beginning of value

Turns out the upstream for these 3 plugins are gone. The plugin json needs to be updated to point to the stable branch copy instead.

Any plugin actions are not affected (Apart from interacting from these 3 plugins ofc)

Discovered by micro-editor/micro#4087