doc: update kernel and glibc reqs for armv6

These binaries are cross-compiled with `x64-gcc-12.3.0-glibc-2.28`
from: https://github.com/rvagg/rpi-newer-crosstools.

Author: kleisauke

BUILDING.md

@@ -133,7 +133,7 @@ welcome.
 | GNU/Linux        | x64              | kernel >= 6.6, musl >= 1.2.4  | e.g. Alpine 3.19            |
 | GNU/Linux        | x86              | kernel >= 3.10, glibc >= 2.17 | Downgraded as of Node.js 10 |
 | GNU/Linux        | armv7            | kernel >= 4.18, glibc >= 2.28 | Downgraded as of Node.js 24 |
-| GNU/Linux        | armv6            | kernel >= 4.14, glibc >= 2.24 | Downgraded as of Node.js 12 |
+| GNU/Linux        | armv6            | kernel >= 4.18, glibc >= 2.28 | Downgraded as of Node.js 12 |
 | GNU/Linux        | loong64          | kernel >= 5.19, glibc >= 2.36 |                             |
 | GNU/Linux        | riscv64          | kernel >= 5.19, glibc >= 2.36 |                             |
 | FreeBSD          | x64              | >= 13.2                       |                             |