doc: promote Alpine Linux to tier 2 support

kernel >= 6.12, musl >= 1.2.5 updated to align with
lowest supported version Alpine Linux 3.21
see https://alpinelinux.org/releases/
see https://alpinelinux.org/posts/Alpine-3.21.0-released.html

Alpine 3.22 listed as example version

Signed-off-by: Stewart X Addison <sxa@ibm.com>
PR-URL: #63737
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Rafael Gonzaga <rafael.nunu@hotmail.com>
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Ruy Adorno <ruy@vlt.sh>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Richard Lau <richard.lau@ibm.com>
Reviewed-By: Ulises Gascón <ulisesgascongonzalez@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Mike McCready <66998419+MikeMcC399@users.noreply.github.com>

Author: sxa

BUILDING.md

@@ -107,7 +107,7 @@ platforms. This is true regardless of entries in the table below.
 | Operating System | Architectures    | Versions                          | Support Type | Notes                                                      |
 | ---------------- | ---------------- | --------------------------------- | ------------ | ---------------------------------------------------------- |
 | GNU/Linux        | x64              | kernel >= 4.18[^1], glibc >= 2.28 | Tier 1       | e.g. Ubuntu 20.04, Debian 10, RHEL 8                       |
-| GNU/Linux        | x64              | kernel >= 3.10, musl >= 1.1.19    | Experimental | e.g. Alpine 3.8                                            |
+| GNU/Linux        | x64              | kernel >= 6.12, musl >= 1.2.5     | Tier 2       | e.g. Alpine 3.22                                           |
 | GNU/Linux        | x86              | kernel >= 3.10, glibc >= 2.17     | Experimental | Downgraded as of Node.js 10                                |
 | GNU/Linux        | arm64            | kernel >= 4.18[^1], glibc >= 2.28 | Tier 1       | e.g. Ubuntu 20.04, Debian 10, RHEL 8                       |
 | GNU/Linux        | armv7            | kernel >= 4.18[^1], glibc >= 2.28 | Experimental | Downgraded as of Node.js 24                                |