* Fix serial speed DD configuration

- Fixes incorrect SOFT_SERIAL_SPEED mapping
- Renames key split.soft_serial_speed -> split.serial.speed
- Migrates keyoards that configure this, and remove configuration from keyboards that do not differ from the default behaviour
- Add deprecation notice and migration support

Migrate split.soft_serial_pin to split.serial.pin

Affects:

  - `qpockets/space_space/rev1`
  - `qpockets/space_space/rev2`
  - `quad_h/lb75`
  - `quantrik/kyuu`
  - `qwertlekeys/calice`
  - `rabbit/rabbit68`
  - `rainkeebs/delilah`
  - `rainkeebs/rainkeeb`
  - `rainkeebs/yasui`
  - `rart/rart45`
  - `rart/rart4x4`
  - `rart/rart60`
  - `rart/rart67`
  - `rart/rart67m`
  - `rart/rart75`
  - `rart/rart75m`
  - `rart/rartand`
  - `rart/rartland`
  - `rart/rartlice`
  - `rart/rartlite`
  - `rart/rartpad`
  - `rate/pistachio/rev1`
  - `rate/pistachio/rev2`
  - `rate/pistachio_mp`
  - `rate/pistachio_pro`
  - `redox/rev1`
  - `redscarf_iiplus/verb`
  - `redscarf_iiplus/verc`
  - `redscarf_iiplus/verd`
  - `retro_75`
  - `reversestudio/decadepad`
  - `reviung/reviung5`
  - `reviung/reviung33`
  - `reviung/reviung34`
  - `reviung/reviung39`
  - `reviung/reviung41`
  - `reviung/reviung53`
  - `rgbkb/zen/rev1`
  - `rgbkb/zen/rev2`
  - `rmi_kb/aelith`
  - `rmi_kb/chevron`
  - `rmi_kb/herringbone/pro`
  - `rmi_kb/herringbone/v1`
  - `rmi_kb/squishy65`
  - `rmi_kb/squishyfrl`
  - `rmi_kb/squishytkl`
  - `rmi_kb/wete/v1`
  - `rmi_kb/wete/v2`
  - `rocketboard_16`
  - `rominronin/katana60/rev1`
  - `rominronin/katana60/rev2`
  - `roseslite`
  - `rotr`
  - `rpiguy9907/southpaw66`
  - `rubi`
  - `rura66/rev1`
  - `ryanbaekr/rb1`
  - `ryanbaekr/rb18`
  - `ryanbaekr/rb69`
  - `ryanbaekr/rb86`
  - `ryanbaekr/rb87`
  - `ryanskidmore/rskeys100`
  - `ryloo_studio/m0110`