configs/imx23evk: bump the U-Boot version

Bump U-Boot to version 2021.10.

This fixes the following error:

/usr/bin/ld: scripts/dtc/dtc-parser.tab.o:(.bss+0x10): multiple definition of `yylloc'; scripts/dtc/dtc-lexer.lex.o:(.bss+0x0): first defined here
collect2: error: ld returned 1 exit status

Fixes:
- https://gitlab.com/buildroot.org/buildroot/-/jobs/1915006170
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

configs/imx23evk_defconfig

@@ -20,7 +20,7 @@ BR2_LINUX_KERNEL_INTREE_DTS_NAME="imx23-evk"
 BR2_TARGET_UBOOT=y
 BR2_TARGET_UBOOT_BOARDNAME="mx23evk"
 BR2_TARGET_UBOOT_CUSTOM_VERSION=y
-BR2_TARGET_UBOOT_CUSTOM_VERSION_VALUE="2019.01"
+BR2_TARGET_UBOOT_CUSTOM_VERSION_VALUE="2021.10"
 BR2_TARGET_UBOOT_FORMAT_SD=y
 
 # Filesystem