update STM32 platform

[avaldebe]

I had not worked on this project for some time and now the STM32f103c8/b environments fail to compile.
This is related to the old platform option

PowerLogger/platformio.ini

Lines 41 to 45 in e5bd2d1

	[env:STM32f103c8]
	; 64 KB flash, RTC: 32768 Hz crystal; display: UC1701 128x64 on SPI2; tact button PA0
	platform = ststm32@<=4.5.0
	debug_tool = stlink
	board = bluepill_f103c8

This version of the platform refers to the Arduino STM32 core by Roger Clark.
Newer versions of the core refer to the STM32duino by ST.

If I remember correctly, I took that version to prevent this issue platformio/platform-ststm32#231.
From the comments, this issue might be avoided by setting board = genericSTM32F103C8.

Using the stm32duino core requires the stm32duino/STM32duino RTC library.