GHA/windows: add minimal mingw-w64 v3 job

Also document mingw-w64 versions for dl-mingw jobs.

Cherry-picked from #18010
Closes #18037

.github/workflows/windows.yml

@@ -420,7 +420,7 @@ jobs:
     strategy:
       matrix:
         include:
-          - name: 'schannel'
+          - name: 'schannel'  # mingw-w64 12.0
             dir: 'mingw64'
             env: 'x86_64'
             ver: '15.0.1'
@@ -428,14 +428,14 @@ jobs:
             config: '-DENABLE_DEBUG=ON -DBUILD_SHARED_LIBS=OFF -DCURL_USE_SCHANNEL=ON -DENABLE_UNICODE=OFF'
             type: 'Release'
             tflags: 'skiprun'
-          - name: 'schannel'
+          - name: 'schannel'  # mingw-w64 10.0
             dir: 'mingw64'
             env: 'x86_64'
             ver: '9.5.0'
             url: 'https://github.com/brechtsanders/winlibs_mingw/releases/download/9.5.0-10.0.0-msvcrt-r1/winlibs-x86_64-posix-seh-gcc-9.5.0-mingw-w64msvcrt-10.0.0-r1.7z'
             config: '-DENABLE_DEBUG=ON -DBUILD_SHARED_LIBS=OFF -DCURL_USE_SCHANNEL=ON -DENABLE_UNICODE=OFF'
             type: 'Release'
-          - name: 'schannel mbedtls U'
+          - name: 'schannel mbedtls U'  # mingw-w64 6.0
             dir: 'mingw64'
             env: 'x86_64'
             ver: '7.3.0'
@@ -444,7 +444,7 @@ jobs:
             install: mingw-w64-x86_64-mbedtls
             type: 'Release'
             tflags: 'skiprun'
-          - name: 'schannel !unity'
+          - name: 'schannel !unity'  # mingw-w64 5.0
             dir: 'mingw32'
             env: 'i686'
             ver: '6.4.0'
@@ -452,6 +452,14 @@ jobs:
             config: '-DENABLE_DEBUG=ON -DBUILD_SHARED_LIBS=OFF -DCURL_USE_SCHANNEL=ON -DENABLE_UNICODE=OFF -DCMAKE_UNITY_BUILD=OFF'
             type: 'Debug'
             tflags: 'skiprun'
+          - name: 'schannel !examples'  # mingw-w64 3.0
+            dir: 'mingw64'
+            env: 'x86_64'
+            ver: '4.8.1'
+            url: 'https://downloads.sourceforge.net/mingw-w64/Toolchains%20targetting%20Win64/Personal%20Builds/mingw-builds/4.8.1/threads-win32/seh/x86_64-4.8.1-release-win32-seh-rt_v3-rev2.7z'
+            config: '-DENABLE_DEBUG=ON -DBUILD_SHARED_LIBS=OFF -DCURL_USE_SCHANNEL=ON'
+            type: 'Debug'
+            tflags: 'skipall'
       fail-fast: false
     steps:
       - uses: msys2/setup-msys2@40677d36a502eb2cf0fb808cc9dec31bf6152638 # v2
@@ -572,6 +580,7 @@ jobs:
           cmake --build bld --target test-ci
 
       - name: 'build examples'
+        if: ${{ !contains(matrix.name, '!examples') }}
         timeout-minutes: 5
         run: |
           PATH="/d/my-cache/${MATRIX_DIR}/bin:$PATH"