Add board specific to Proton-C, with usual defaults turned on. (#10976)

- Set all other ChibiOS defaults to 'off', when not targeting Proton-C
- Modified all existing F303 boards to point at the QMK_PROTON_C to ensure repeatable binary output
- Modified version.h generation so that SKIP_VERSION=yes generates the same output

docs/ja/proton_c_conversion.md

@@ -51,6 +51,7 @@ Proton C には1つのオンボード LED(C13)しかなく、デフォルトで
 
 ```
 MCU = STM32F303
+BOARD = QMK_PROTON_C
 ```
 
 次の変数が存在する場合は削除します。

docs/proton_c_conversion.md

@@ -44,6 +44,7 @@ To use the Proton C natively, without having to specify `CTPC=yes`, you need to
 
 ```
 MCU = STM32F303
+BOARD = QMK_PROTON_C
 ```
 
 Remove these variables if they exist: