CI: remove windows-2019 tests (#21511)

Github has removed the windows-2019 runners and all actions using them
automatically fail with the message:

Windows Server 2019 has been retired. The Windows Server 2019 image has
been removed as of 2025-06-30. For more details, see
actions/runner-images#12045

Signed-off-by: Andrei Horodniceanu <[email protected]>

Author: the-horo

.github/workflows/runnable_cxx.yml

@@ -77,8 +77,6 @@ jobs:
           # NOTE: as of April 2025, image windows-2025 only has VS 2022, so no point in testing that image too
           - { os: windows-2022, model: 64 }
           - { os: windows-2022, model: 32 }
-          - { os: windows-2019, model: 64 }
-          - { os: windows-2019, model: 32 }
 
     runs-on: ${{ matrix.os }}
     defaults: