configs/lego_ev3: update to U-Boot 2021.07

Buildroot CI has been failing due to 404 error on older U-Boot. Updating
the U-Boot version should fix it and it doesn't hurt to use the latest
U-Boot anyway.

Signed-off-by: David Lechner <david@lechnology.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

configs/lego_ev3_defconfig

@@ -43,7 +43,7 @@ BR2_TARGET_ROOTFS_SQUASHFS4_XZ=y
 BR2_TARGET_UBOOT=y
 BR2_TARGET_UBOOT_BUILD_SYSTEM_KCONFIG=y
 BR2_TARGET_UBOOT_CUSTOM_VERSION=y
-BR2_TARGET_UBOOT_CUSTOM_VERSION_VALUE="2018.11"
+BR2_TARGET_UBOOT_CUSTOM_VERSION_VALUE="2021.07"
 BR2_TARGET_UBOOT_BOARD_DEFCONFIG="legoev3"
 
 # host