configs/friendlyarm_nanopi_r3s: fix host linux headers custom version

Since its introduction in commit [1], the
friendlyarm_nanopi_r3s_defconfig is failing to build with error:

    Incorrect selection of kernel headers: expected 6.12.x, got 6.13.x

The error happens because the defconfig has:
BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_6_12=y
and
BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="6.13.4"

This commit fixes the issue by setting instead:
BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="6.12.28"

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/9887216429

[1] https://gitlab.com/buildroot.org/buildroot/-/commit/41734e00c0cc36f4df2f492e72b535a0716d4048

Cc: Sergey Kuzminov <kuzminov.sergey81@gmail.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>
(cherry picked from commit 74c4dd41502384a797829ae16167c28f50a24e3e)
[Thomas:
  Since this has been introduced in LTS branch I set the kernel
  version to 6.12.x present in LTS instead of changing the linux header
  version.
]
Signed-off-by: Thomas Perale <thomas.perale@mind.be>

configs/friendlyarm_nanopi_r3s_defconfig

@@ -11,7 +11,7 @@ BR2_ROOTFS_POST_IMAGE_SCRIPT="support/scripts/genimage.sh"
 BR2_ROOTFS_POST_SCRIPT_ARGS="-c $(BINARIES_DIR)/genimage.cfg"
 BR2_LINUX_KERNEL=y
 BR2_LINUX_KERNEL_CUSTOM_VERSION=y
-BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="6.13.4"
+BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="6.12.28"
 BR2_LINUX_KERNEL_USE_CUSTOM_CONFIG=y
 BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE="board/friendlyarm/nanopi-r3s/kernel.config"
 BR2_LINUX_KERNEL_DTS_SUPPORT=y