toolchain: gcc: switch default to 13

toolchain: gcc: switch default to 13

Use GCC 13 instead of GCC 12 by default.

All target kernels are building with GCC 13.
Most packages from the feed are building fine.

The root file systems is getting a little bit smaller for MIPS 32 BE
and aarch64.

With GCC 12 I got these sizes for lantiq/xrx200:
7,005,867 openwrt-lantiq-xrx200-tplink_tdw8970-initramfs-kernel.bin

With GCC 13 I got these sizes for lantiq/xrx200:
6,989,754 openwrt-lantiq-xrx200-tplink_tdw8970-initramfs-kernel.bin

With GCC 12 I got these sizes for armsr/armv8:
13,083,836 openwrt-armsr-armv8-generic-ext4-combined.img.gz
 4,900,240 openwrt-armsr-armv8-generic-ext4-rootfs.img.gz
20,142,592 openwrt-armsr-armv8-generic-kernel.bin

With GCC 13 I got these sizes for armsr/armv8:
13,068,966 openwrt-armsr-armv8-generic-ext4-combined.img.gz
 4,893,078 openwrt-armsr-armv8-generic-ext4-rootfs.img.gz
20,142,592 openwrt-armsr-armv8-generic-kernel.bin

Signed-off-by: Nick Hainke <vincent@systemli.org>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>