BastardKB: upstream charybdis/4x6

[finrod09]

Description

This change aims to upstream several changes accumulated in the BastardKB fork. It is submitted alondside several companion PRs, which divide the very similar set of changes by board. Changes requested during review of the initial draft have been appied here as well, where applicable.

The full set of related PRs is:

• charybdis/3x5 BastardKB: Upstream charybdis/3x5 #23638
• charybdis/3x6 BastardKB: upstream charybdis/3x6 #23810
• charybdis/4x6 BastardKB: upstream charybdis/4x6 #23811
• skeletyl BastardKB: upstream skeletyl #23812
• tbkmini BastardKB: upstream tbkmini #23813
• scylla BastardBK: upstream scylla #23814
• dilemma/3x5_2 BastardKB: upstream dilemma/3x5_2 #23815
• dilemma/3x5_3 BastardKB: upstream dilemma/3x5_3 #23817
• dilemma/4x6_4 BastardKB: upstream dilemma/4x6_4 #23816
• tbk (removal) BastardKB: remove legacy board tbk #23818

Description from the inital PR repeated for convenience:

The most extensive of these changes is dropping support for all controllers and shields other than v2/splinky_3, which has been flattened into the main keyboard. Older variants have not been sold in kits or prebuilts nor been recommended to self-source for a long time.

The dates GH shows on these commits is misleading as some rewriting of the history was necessary to separate commits by keyboard; The functional changes by @0xcharly have been shipping with kits and prebuilts for months and are well-tested. Changes up to and including 3799cd5 are live downstream.

Care has been taken to not lose changes introduced by #23622 and #23627.

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).

[drashna]

Though, honestly, I would kind of opt for the blackpill version to stay, as I'm actually using it.