Migrate off of set-output, which is deprecated

This change eliminates warnings in CI caused by the deprecation.

In addition, using the `python-version` output of the
`setup-python` action fixes a bug that prevented Windows caches
from containing the Python version.

For reference, Windows caches were created with names
that lacked the Python version, like:

```
venv-pull_request-Windows--{hash}
```

Author: kurtmckee

.github/workflows/build-docs.yml

@@ -12,14 +12,11 @@ jobs:
       - uses: actions/checkout@v4
 
       - name: Set up Python 3.12
+        id: setup-python
         uses: actions/setup-python@v5
         with:
           python-version: 3.12
 
-      - name: Get full Python version
-        id: full-python-version
-        run: echo ::set-output name=version::$(python -c "import sys; print('-'.join(str(v) for v in sys.version_info))")
-
       - name: Set up poetry
         uses: Gr1N/setup-poetry@v9
 
@@ -31,7 +28,7 @@ jobs:
         id: cache
         with:
           path: .venv
-          key: venv-${{ steps.full-python-version.outputs.version }}-${{ hashFiles('**/poetry.lock') }}
+          key: venv-${{ steps.setup-python.outputs.python-version }}-${{ hashFiles('**/poetry.lock') }}
 
       - name: Ensure cache is healthy
         if: steps.cache.outputs.cache-hit == 'true'

.github/workflows/python-test.yml

@@ -21,14 +21,11 @@ jobs:
       - uses: actions/checkout@v4
 
       - name: Set up Python ${{ matrix.python-version }}
+        id: setup-python
         uses: actions/setup-python@v5
         with:
           python-version: ${{ matrix.python-version }}
 
-      - name: Get full Python version
-        id: full-python-version
-        run: echo ::set-output name=version::$(python -c "import sys; print('-'.join(str(v) for v in sys.version_info))")
-
       - name: Set up poetry
         uses: Gr1N/setup-poetry@v9
 
@@ -40,7 +37,7 @@ jobs:
         id: cache
         with:
           path: .venv
-          key: venv-${{ github.event_name }}-${{ runner.os }}-${{ steps.full-python-version.outputs.version }}-${{ hashFiles('**/poetry.lock') }}
+          key: venv-${{ github.event_name }}-${{ runner.os }}-${{ steps.setup-python.outputs.python-version }}-${{ hashFiles('**/poetry.lock') }}
 
       - name: Ensure cache is healthy
         if: steps.cache.outputs.cache-hit == 'true'