ci: remove allow-prereleases for Python 3.14

allow-prereleases is not longer needed for Python 3.14.

Author: TomJGooding

.github/workflows/pythonpackage.yml

@@ -31,7 +31,6 @@ jobs:
         uses: actions/setup-python@v5
         with:
           python-version: ${{ matrix.python-version }}
-          allow-prereleases: true
           cache: "poetry"
       - name: Install dependencies
         run: poetry install --no-interaction