Update OpenSSH_Server_Configuration.md

GSSAPIAuthentication was added with *OpenSSH for Windows 8.1* which came in “2021-05 Cumulative Update for Windows 10” (KB5003173), which is available for Windows 10 Version 20H1 or newer.
Source: PowerShell/Win32-OpenSSH#1693

GSSAPIAuthentication only supports Kerberos (i.e., not SPNEGO or NTLM authentication), so it is useful to mention Kerberos explicitly here.
Source: PowerShell/Win32-OpenSSH#1295

WindowsServerDocs/administration/OpenSSH/OpenSSH_Server_Configuration.md

@@ -131,7 +131,7 @@ scp with chroot, by implementing a custom shell that would only allow scp and sf
 
 ### GSSAPIAuthentication
 
-The `GSSAPIAuthentication` configuration argument specifies whether GSSAPI based user authentication is allowed. The
+The `GSSAPIAuthentication` configuration argument specifies whether GSSAPI (Kerberos) based user authentication is allowed. The
 default for `GSSAPIAuthentication` is no.
 
 GSSAPI authentication also requires the use of the `-K` switch specifying the hostname when using
@@ -152,7 +152,7 @@ Host SERVER01.contoso.com
 ```
 
 > [!IMPORTANT]
-> GSSAPI is only available starting in Windows Server 2022, Windows 11, and Windows 10 xxxx.
+> GSSAPI is only available starting in Windows Server 2022, Windows 11, and Windows 10 (May 2021 Update).
 
 ### HostKey