AzerothCore not up to date with joiner

@Yehonal I ran the installer.sh in azerothcore, so it updated the joiner but the readme is still a very old

So I looked at the files and at the commits

Apparently, we have this in AzerothCore:

```
J_VER_REQ="v0.8.3"
J_PATH="$AC_PATH_APPS/drassil/joiner"
J_PATH_MODULES="$AC_PATH_MODULES"
```

And the new version of the joiner is 0.8.4 hence why the old readme

Start by comparing installer.sh with the README and inspect the J_VER_REQ value shown in the issue. Confirm whether the documentation still describes joiner v0.8.3 while the installer uses the newer v0.8.4, then update the relevant README information so it matches the current joiner version.

Written by the indexing model from the issue text.