ci: build Linux releases on Ubuntu 20.04 for compatibility

Change the Linux build platform from Ubuntu 22.04 to Ubuntu 20.04 so release packages target glibc 2.31 and install on Ubuntu 20.04.

README_EN.md

@@ -32,7 +32,7 @@ CrossDesk is an experimental application of [MiniRTC](https://github.com/kunkund
 |-----------|-----------------|
 | **Windows** | Windows 10 or later (64-bit) |
 | **macOS** | macOS Intel 15.0 or later *(versions between 14.0 and 15.0 can be built manually for compatibility)*<br>macOS Apple Silicon 14.0 or later |
-| **Linux** | Ubuntu 22.04 or later *(older versions can be built manually for compatibility)* |
+| **Linux** | Ubuntu 20.04 or later |
 
 
 ## Usage
@@ -85,27 +85,41 @@ If the remote Windows service is not installed, not running, or temporarily unav
 
 Requirements:
 - [xmake](https://xmake.io/#/guide/installation)
-- [cmake](https://cmake.org/download/)
+- [cmake](https://cmake.org/download/) 3.21 or later
 
 ### Linux
 
-Linux builds currently support Ubuntu 22.04 or later on amd64 and arm64. Install the base build dependencies first:
+Linux builds support Ubuntu 20.04 or later on amd64 and arm64. Release packages
+use Ubuntu 20.04/glibc 2.31 as their compatibility baseline. Install the base
+build dependencies first:
 
 ```bash
 sudo apt-get update
 sudo apt-get install -y \
-  git curl unzip build-essential cmake pkg-config binutils dpkg-dev \
+  git curl unzip build-essential gcc-10 g++-10 python3-pip \
+  pkg-config binutils dpkg-dev \
   libx11-dev libxext-dev libxrender-dev libxft-dev libxrandr-dev \
   libxinerama-dev libxcursor-dev libxi-dev libxfixes-dev libxv-dev \
   libxtst-dev libxcb-randr0-dev libxcb-xtest0-dev libxcb-xinerama0-dev \
   libxcb-shape0-dev libxcb-xkb-dev libxcb-xfixes0-dev libxcb-shm0-dev \
   libasound2-dev libsndio-dev libpulse-dev
+
+# Ubuntu 20.04's CMake 3.16 is too old for Slint 1.17.
+python3 -m pip install --user cmake==3.31.6
+export PATH="$HOME/.local/bin:$PATH"
 ```
 
+Ubuntu 20.04's default GCC 9 lacks parts of the C++20 standard library needed
+by Slint's C++ API, so the commands above install GCC 10 and the configuration
+below selects it explicitly.
+
 Install the optional dependencies when Wayland capture or DRM capture is enabled:
 
 ```bash
-sudo apt-get install -y libdbus-1-dev libpipewire-0.3-dev libdrm-dev
+sudo apt-get install -y libdbus-1-dev libdrm-dev
+
+# Ubuntu 22.04 and later can alternatively use the distribution packages.
+# sudo apt-get install -y libpipewire-0.3-dev libspa-0.2-dev
 ```
 
 Clone the submodules and build a release binary:
@@ -117,7 +131,12 @@ cd crossdesk
 # Run this when the repository has already been cloned
 git submodule update --init --recursive
 
-xmake f -m release --USE_CUDA=false -y
+# Ubuntu 20.04 has no PipeWire 0.3 development package. Install the
+# header-only SDK used during compilation. Skip this when the distribution's
+# PipeWire 0.3 development packages are already installed.
+sudo ./docker/linux-build/install-pipewire-sdk.sh
+
+xmake f -m release --USE_CUDA=false --cc=gcc-10 --cxx=g++-10 -y
 xmake b -vy crossdesk
 ```
 
@@ -126,7 +145,8 @@ The amd64 binary is written to `build/linux/x86_64/release/crossdesk`; the arm64
 Example configuration with Wayland and DRM capture enabled:
 
 ```bash
-xmake f -m release --USE_WAYLAND=true --USE_DRM=true --USE_CUDA=false -y
+xmake f -m release --USE_WAYLAND=true --USE_DRM=true --USE_CUDA=false \
+  --cc=gcc-10 --cxx=g++-10 -y
 xmake b -vy crossdesk
 ```
 
@@ -141,6 +161,9 @@ Build a Debian package after compiling a release binary on the matching architec
 ```
 
 The package scripts install the shared Slint runtime in the private `/usr/lib/crossdesk` directory, so users do not need to install `libslint_cpp.so` separately.
+PipeWire is not a mandatory runtime dependency. CrossDesk detects the host's
+PipeWire 0.3 runtime dynamically and can still use X11 capture (and DRM when
+enabled at build time) when it is unavailable.
 
 ### Common build options
 
@@ -163,8 +186,8 @@ xmake r crossdesk
 
 #### Development Without CUDA Environment
 
-For **Linux developers who do not have a CUDA environment installed and want to enable hardware codec feature**, a preconfigured [Ubuntu 22.04 Docker image](https://hub.docker.com/r/crossdesk/ubuntu22.04) is provided.  
-This image comes with all required build dependencies and allows you to build the project directly inside the container without any additional setup.
+For **Linux developers who do not have a CUDA environment installed and want to enable hardware codec feature**, a preconfigured [Ubuntu 20.04 compatibility build image](https://hub.docker.com/r/crossdesk/ubuntu20.04) is provided.
+This image contains the required build dependencies and produces a single Linux package compatible with the glibc 2.31 baseline.
 
 After entering the container, download the project and run:
 ```