To have discs in the same folder

I do not care. I have one in media and one in srv.

If you want to have the one which is now in media in srv, one option would be to remove all references and unmount the drive. If you mount the drive after that it should be mounted in srv. I have not tested this, though.

It's a matter of default locations.

It seems as if you set up a data drive on OMV 3, before version 3.0.42 (I think). Back then, by default, new data drives were installed in /media with a long series of numbers for a unique ID. Now, in later versions of OMV3 and in 4, the default location for a data drive is /srv with a name derived from the disks label. (Your newer second drive.)

In the bottom line, it doesn't matter where a drive is mounted. (I have a ZFS array mounted at /ZFS1, at the root of the boot drive.) While you can do as you like, moving a drive from /media to /srv will result in a lot of unnecessary work. You'd have to reconfigure shares, Samba, etc. I don't know of any short cuts.