Merge bitcoin#28900: doc: remove mingw-w64 install for "older" systems

656a7e9 doc: remove mingw-w64 install for "older" systems (fanquake)

Pull request description:

  Now that we require GCC 10.1+, the posix variant is available on supported systems.

  i.e:
  https://packages.debian.org/bullseye/g++-mingw-w64-x86-64-posix
  https://packages.ubuntu.com/jammy/g++-mingw-w64-x86-64-posix

ACKs for top commit:
  maflcko:
    ACK 656a7e9
  BrandonOdiwuor:
    ACK 656a7e9
  hebasto:
    ACK 656a7e9.

Tree-SHA512: b7d3696ad5d0322c107ced5750d20c40167caaf7d063cf01da5fc12c4086827f4f73185aa5cc9ac170778b0523c0c16cca3b2419b11019da9d30b936ee897e14

doc/build-windows.md

@@ -48,16 +48,9 @@ Acquire the source in the usual way:
 ## Building for 64-bit Windows
 
 The first step is to install the mingw-w64 cross-compilation tool chain:
-  - on modern systems (Ubuntu 21.04 Hirsute Hippo or newer, Debian 11 Bullseye or newer):
 
 ```sh
 sudo apt install g++-mingw-w64-x86-64-posix
-```
-
-  - on older systems:
-
-```sh
-sudo apt install g++-mingw-w64-x86-64
 ```
 
 Once the toolchain is installed the build steps are common: