Keep external SdFat version at 2.1.2.

Upstream SdFat 2.2 has some incompatible API changes.
Keep the version we use in sync with the SdFat_NoArduino fork.

(cherry picked from commit e7c8fe7293276957b494ffa533fe4c7c2e954ff2)

platformio.ini

@@ -69,7 +69,7 @@ framework = arduino
 board = ZuluSCSI_RP2040
 board_build.ldscript = lib/ZuluSCSI_platform_RP2040/rp2040.ld
 lib_deps =
-    SdFat=https://github.com/greiman/SdFat
+    SdFat=https://github.com/greiman/SdFat#2.1.2
     minIni
     ZuluSCSI_platform_RP2040
     SCSI2SD