Unable to run su in adb shell on an Ubuntu machine
Obviously the su binary you installed does not work well with your system. As you luckily created a backup of the original su executable, recommendation is to revert the incompatible changes first:

adb shell
$ oldsu
# mount -o remount,rw /system
# rm /system/xbin/su /system/bin/su
# cp /system/xbin/oldsu /system/xbin/su
# exit
$ exit
adb reboot


Explanation: Start adb shell and become root (using the working oldsu executable). Delete /system/xbin/su (which is only a symlink now) and the incompatible /system/bin/su, then copy the working su back to where it belongs. Then exit the root shell and finally adb shell. The reboot is optional, and should not be needed. Also optionally you could remove busybox (or keep it if it works). Before exiting adb shell you might wish to test whether su works again, just to be sure (which is why I wrote cp for this: if anything goes wrong, you still have your oldsu available).

With those steps, you should at least have everything in working order again, and this issue is solved. Updating to a newer Android version is a separate issue: Please check the update tag, as corresponding questions already exist. Same for the ROM you might need (check the rom tag for that, and in also check the tag-wikis for update and rom).