STM32F103C8T6 ADC value problem
I agree with analogsystemsrf that the power supply of the MCU is the likely problem. But there are more problems than mentioned.
Have a look at Application Note AN2586 (Getting started with STM32F10xxx hardware development). It clearly outlines how the power supply should be designed. Several issues should be improved:

Decoupling capacitors should be close to each power supply pin of the MCU
Analog supply should have a separate set of decoupling capacitors
Both the analog and the digital supply should have an additional bigger capacitor (1µF or 4.7µF in addition to the several 100nF ones)
The board should have a ground plane
The AMS1117 output capacitor (1µF) is at the lower limit

Update
As an example for a STM32F103C8 design with components on a single side only, see the below board.
I don't have the schematic or PCB layout but I'm quite confident the capacitors near the corners of the STM32 MCU are the decoupling capacitors. The component near the pins A2/A3 might even be a ferrite bead for the analog supply.