Refs: #61449

Situation

The Supported toolchains section of the BUILDING.md document specifies:

1. The use of only a lower bound for the version of Visual Studio conflicts with the structure of the Windows vcbuild.bat. Each version of Visual Studio needs to be explicitly supported by the script, which currently includes the ability to work with Visual Studio 2022 and 2026 only. According to Visual Studio Product Lifecycle and Servicing it can be expected that Microsoft will release Visual Studio 2027 in November 2026 (See also https://learn.microsoft.com/en-us/visualstudio/releases/2026/release-rhythm#annual-releases). The script will not automatically support this version and the statement of compatibility with "Visual Studio >= 2022" will become incorrect.
2. As described in the Windows > Option 1: Manual install section of the BUILDING.md document, the "Desktop development with C++" workload is required. In the older Visual Studio 2019, this installed the Windows 10 SDK. The Visual Studio 2022 & 2026 releases however install the Windows 11 SDK, not the Windows 10 SDK by default, although they do allow optionally installing the Windows 10 SDK.
3. Windows 10 Home & Pro reached end-of-life on Oct 14, 2025. Windows 10 Enterprise LTSC 2019 has a future end-of-life on Jan 9, 2029. Windows 10 LTSC states:

   The Long-Term Servicing Channel (LTSC) is designed to be used only for specialized devices - for example, those that control medical equipment or automated teller machines (ATMs).

Change

Update the Supported toolchains section of the BUILDING.md document to:

Backporting

• This PR can be backported to v25.x with no restrictions

• Backporting to v24.x requires the landing of PR [v24.x backport] build: add support for Visual Studio 2026 #61840 for consistency between documentation and capabilities, although backporting without the other PR won't make the documentation any more inaccurate than it currently is

• Do not backport to v22.x or v20.x since they are not compatible with Visual Studio 2026 (see [v20.x] doc: supported toolchain with Visual Studio 2019 & 2022 only #61450 & [v22.x] doc: supported toolchain with Visual Studio 2022 only #61451 instead)