Feature: Blackpill-F4 ADC

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Detailed description

• This is a new feature for one platform, enhancing feature-parity with other platforms.
• The existing problem is blackpill-f4 measuring "Unknown" target voltage.
• This PR solves that by providing an implementation of platform_target_voltage() through ADC1 channel 8 pin PB0 expecting 1/2 resistive divided VTRef (routed by external add-on board, for example).

Idea implemented in November of 2024. Not runtime-tested since then because I don't have add-on boards.

Your checklist for this pull request

• I've read the Code of Conduct
• I've read the guidelines for contributing to this repository
• It builds for hardware native (see Building the firmware)
• It builds as BMDA (see Building the BMDA)
• I've tested it to the best of my ability
• My commit messages provide a useful short description of what the commits do

Closing issues