Python version not updated because the image build is broken by a runtime dependency issue. The fix #40959 can't be backported retroactively to the tag.

Closes #40975

COPYBARA_INTEGRATE_REVIEW=#40975 from sergiitk:interop-1.76.0 cd18cff
PiperOrigin-RevId: 826173165

Pip released [v25.3](https://pip.pypa.io/en/stable/news/#v25-3) a few days back which doesn't support `setup.py` builds anymore, while we are yet to migrate to use the `pyproject.toml` build system in grpc#40833.

Hence some of our Python tests using the most recent versions of pip for the build have started to fail with errors like:
```
  ERROR: Failed building wheel for grpcio
Failed to build grpcio
error: failed-wheel-build-for-install

× Failed to build installable wheels for some pyproject.toml based projects
```

This PR hence pins the pip version to 25.2 which still supports setup.py build until grpc#40833 is submitted.

Closes grpc#40959

COPYBARA_INTEGRATE_REVIEW=grpc#40959 from sreenithi:pin_pip_version d72e056
PiperOrigin-RevId: 824584176

Python version not updated because the image build is broken by a runtime dependency issue. The fix grpc#40959 can't be backported retroactively to the tag.

Closes grpc#40975

COPYBARA_INTEGRATE_REVIEW=grpc#40975 from sergiitk:interop-1.76.0 cd18cff
PiperOrigin-RevId: 826173165