Remove bootloaders for MG21, add for BRD2709A (#191)

* Remove bootloaders for MG21, add for BRD2709A

* Fix typo in README for bootloader binaries

Author: selissia

board-support/efr32/bootloader_binaries/README.md

@@ -1,11 +1,7 @@
 # Bootloader binaries for Matter
 
-This directory contains Silicon Labs Gecko Bootloader binaries that can be used with Matter OTA Software Update functionality.
+This directory contains Silicon Labs Gecko Bootloader binaries that can be used with Matter OTA Software Update functionality. The binaries are provided here for historical purposes. The recommended way of obtaining bootloader binaries is by building the Matter Solutions with Studio or SLC CLI. 
 
 - The external storage (SPI flash) bootloaders are configured with the slot size of 1024K 
-- The internal storage bootloaders (supported on MG24 cards only) are configured as follows:
-  - Slot Address: 135176192 (0x080ea000)
-  - Slot Size: 557056
 - All bootloaders supports LZMA compression
-- For MG12 boards use the <name>-combined.s37 (e.g. bootloader-storage-spiflash-single-1024k-BRD4161A-gsdk4.1-combined.s37) bootloader version the first time a device is programmed, whether during development or manufacturing. For subsequent programming, if the "combined" bootloader had been previously flashed to the device use the regular version (<name>.s37, e.g. bootloader-storage-spiflash-single-1024k-BRD4161A-gsdk4.1.s37). 
-- The bootloaders are compatible with Matter applications built with GSDK 4.1. Bootloaders for Matter applications built with GSDK 3.1 can be found here: https://github.com/SiliconLabs/sdk_support/tree/cf743ca3cca307ddb209c45f62aed7dc5d8ce06d/platform/bootloader/sample-apps/bootloader-storage-spiflash-single .
+- The bootloaders are compatible with Matter applications built with GSDK 4.1 or later. Bootloaders for Matter applications built with GSDK 3.1 can be found here: https://github.com/SiliconLabs/sdk_support/tree/cf743ca3cca307ddb209c45f62aed7dc5d8ce06d/platform/bootloader/sample-apps/bootloader-storage-spiflash-single .