build: remove implied support for win 2012 not R2

PR-URL: #19378
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Nikolai Vavilov <vvnicholas@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

BUILDING.md

@@ -42,7 +42,7 @@ platforms in production.
 | GNU/Linux    | Tier 1       | kernel >= 2.6.32, glibc >= 2.12  | x64, arm             |                  |
 | GNU/Linux    | Tier 1       | kernel >= 3.10, glibc >= 2.17    | arm64                |                  |
 | macOS        | Tier 1       | >= 10.10                         | x64                  |                  |
-| Windows      | Tier 1       | >= Windows 7/2008 R2             | x86, x64             | vs2017           |
+| Windows      | Tier 1       | >= Windows 7/2008 R2/2012 R2     | x86, x64             | vs2017           |
 | SmartOS      | Tier 2       | >= 15 < 16.4                     | x86, x64             | see note1        |
 | FreeBSD      | Tier 2       | >= 10                            | x64                  |                  |
 | GNU/Linux    | Tier 2       | kernel >= 3.13.0, glibc >= 2.19  | ppc64le >=power8     |                  |