What will happen if I feed a UA741 op-amp lower negative supply voltage than recommended?
If you put 12 volts across the positive and negative supply inputs of the op amp, it's going to operate the same regardless of whether you choose to call those values +6 and -6, +12 and 0, or +8 and -4. The 741 can only output about 3 volts from each rail. (You can find this in the data sheet by noting that the maximum output voltage with a +15V positive supply is +12V, and similarly on the negative side.) You said you don't need negative output, so splitting your supply into +8V and -4V, or +9V and -3V, would give you the 0-5V output range you need.

Or get a different op amp. The TI 2471/2/4 for example will run on a 5V single supply and can output within a few tens of millivolts of both rails.