bump minimum platform support to Windows 10, macOS 10.15

We upgraded to Go 1.22 a while ago:

https://go.dev/wiki/MinimumRequirements

> For Go 1.21 and later: Windows 10 and higher or Windows Server 2016
> and higher.

> Go 1.21 and later will support macOS Catalina 10.15 or newer; see https://go.dev/doc/go1.20#darwin.

See also:

- golang/go#57003
- golang/go#57125
- golang/go#64622 (comment)

Windows 7 has oficially been EOL'd in Jan 2020, almost 5 years ago. It
is reasonable to not officially support it.

docs/BUILD.md

@@ -6,8 +6,8 @@ Wallet application.
 * Debian: 11 bullseye or newer
 * Ubuntu: 20.04+
 * Fedora: 36+
-* MacOS: 10.13+
-* Windows: Windows 7+
+* MacOS: 10.15+
+* Windows: Windows 10+
 
 ## Debian, Ubuntu, and Fedora GNU/Linux with Docker
 

env.mk.inc

@@ -1,2 +1,2 @@
 LIBNAME=libserver
-MACOS_MIN_VERSION := 10.11
+MACOS_MIN_VERSION := 10.15