CI: drop windows-2019 jobs

The images will be removed at the end of the month:
https://github.com/actions/runner-images/issues/12045

.ci/Dockerfile.2019

@@ -1,13 +0,0 @@
-FROM mcr.microsoft.com/windows/servercore:ltsc2019
-
-COPY ./msys2-base-x86_64-latest.sfx.exe /msys2.exe
-
-RUN powershell -Command \
-  $ErrorActionPreference = 'Stop'; \
-  $ProgressPreference = 'SilentlyContinue'; \
-  /msys2.exe -y -oC:\; \
-  function msys() { C:\msys64\usr\bin\bash.exe @('-lc') + @Args; } \
-  msys ' '; \
-  msys 'pacman --noconfirm -Syuu'; \
-  msys 'pacman --noconfirm -Syuu'; \
-  msys 'pacman --noconfirm -Scc';

.github/workflows/build.yml

@@ -90,7 +90,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        image: [2019, 2022, 2025]
+        image: [2022, 2025]
     steps:
       - name: Checkout code
         uses: actions/checkout@v4
@@ -128,7 +128,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        image: [2019, 2022, 2025]
+        image: [2022, 2025]
     steps:
       - name: Checkout code
         uses: actions/checkout@v4
@@ -163,7 +163,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        image: [2019, 2022, 2025, 11-arm]
+        image: [2022, 2025, 11-arm]
     steps:
       - name: Checkout code
         uses: actions/checkout@v4