You can try to just Restore the lastest MC18 backup into MC23.
https://wiki.jriver.com/index.php/Library_BackupYou may see issues though because I think there were some changes in MC19 or so that may mean the restore won't completely work. MC23 can restore backups from MC20, MC21, and MC23 no problem, but with MC18 and MC19 might miss something. Some have reported that an MC19 backup works fine in MC23 though. Try it and see.
If it doesn't work properly, then the process required is to download MC19 and install it as a trial, then restore the MC18 library into it, then make a backup of MC19. Then restore that to MC23.
If that still doesn't work properly, then download MC20 and install it as a trial, then restore the MC19 library into it, then make a backup of MC20. Then restore that to MC23.
Old versions of MC are available for download in this forum:
https://yabb.jriver.com/interact/index.php/board,20.0.html