STM32F1: restore latest ststm32 version (#15776)

platformio.ini

 # STM32F103RE
 #
 [env:STM32F103RE]
-platform      = ststm32@5.6.0
+platform      = ststm32
 framework     = arduino
 board         = genericSTM32F103RE
 build_flags   = !python Marlin/src/HAL/HAL_STM32F1/build_flags.py
 # STM32F103RC_fysetc
 #
 [env:STM32F103RC_fysetc]
-platform          = ststm32@5.6.0
+platform          = ststm32
 framework         = arduino
 board             = genericSTM32F103RC
 #board_build.core = maple
 # BigTree SKR Mini V1.1 / SKR mini E3 / SKR E3 DIP (STM32F103RCT6 ARM Cortex-M3)
 #
 [env:STM32F103RC_bigtree]
-platform          = ststm32@5.6.0
+platform          = ststm32
 framework         = arduino
 board             = genericSTM32F103RC
 platform_packages = tool-stm32duino
 monitor_speed     = 115200
 
 [env:STM32F103RC_bigtree_USB]
-platform          = ststm32@5.6.0
+platform          = ststm32
 framework         = arduino
 board             = genericSTM32F103RC
 platform_packages = tool-stm32duino
 # Longer 3D board in Alfawise U20 (STM32F103VET6)
 #
 [env:STM32F103VE_longer]
-platform      = ststm32@5.6.0
+platform      = ststm32
 framework     = arduino
 board         = genericSTM32F103VE
 monitor_speed = 250000
 # MKS Robin (STM32F103ZET6)
 #
 [env:mks_robin]
-platform      = ststm32@5.6.0
+platform      = ststm32
 framework     = arduino
 board         = genericSTM32F103ZE
 extra_scripts = buildroot/share/PlatformIO/scripts/mks_robin.py
 # MKS ROBIN LITE/LITE2 (STM32F103RCT6)
 #
 [env:mks_robin_lite]
-platform      = ststm32@5.6.0
+platform      = ststm32
 framework     = arduino
 board         = genericSTM32F103RC
 extra_scripts = buildroot/share/PlatformIO/scripts/mks_robin_lite.py
 # MKS Robin Mini (STM32F103VET6)
 #
 [env:mks_robin_mini]
-platform      = ststm32@5.6.0
+platform      = ststm32
 framework     = arduino
 board         = genericSTM32F103VE
 extra_scripts = buildroot/share/PlatformIO/scripts/mks_robin_mini.py
 # MKS Robin Nano (STM32F103VET6)
 #
 [env:mks_robin_nano]
-platform      = ststm32@5.6.0
+platform      = ststm32
 framework     = arduino
 board         = genericSTM32F103VE
 extra_scripts = buildroot/share/PlatformIO/scripts/mks_robin_nano.py
 # JGAurora A5S A1 (STM32F103ZET6)
 #
 [env:jgaurora_a5s_a1]
-platform      = ststm32@5.6.0
+platform      = ststm32
 framework     = arduino
 board         = genericSTM32F103ZE
 extra_scripts = buildroot/share/PlatformIO/scripts/jgaurora_a5s_a1_with_bootloader.py
 # Chitu boards like Tronxy X5s (STM32F103ZET6)
 #
 [env:chitu_f103]
-platform      = ststm32@5.6.0
+platform      = ststm32
 framework     = arduino
 board         = genericSTM32F103ZE
 extra_scripts = buildroot/share/PlatformIO/scripts/chitu_crypt.py