Swap F4x1 default board files away from blackpill

[zvecr]

Description

Going forward this aligns the defaults with other STM32 variants. Blackpill users should take advantage of the DD info.json config:

{
  "development_board": "blackpill_f401"
}

or

{
  "development_board": "blackpill_f411"
}

Which will correctly set up the mcu / bootlaoder / board files.

# Find keyboards to migrate
rg -l "MCU = STM32F4.1" keyboards/ | xargs grep -L "^BOARD"

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).