poetry: create a CI with Debian stable's version of poetry.

This is mainly for our own information. It doesn't have to be a required check.

.github/workflows/build.yml

@@ -79,6 +79,26 @@ jobs:
       - name: Check that `mkdocs` can build the docs
         run: poetry run -- mkdocs build --strict
 
+  mkdocs-old-poetry:
+    name: Check that MkDocs can build the docs with Poetry 1.3.2
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11
+      - uses: actions/setup-python@v4
+        with:
+          python-version: 3.11
+      - name: Install poetry
+        uses: abatilo/actions-poetry@v2
+        with:
+          # Test with the version of Poetry in Debian stable. If this starts
+          # failing, we should increase this version and document the minimum
+          # necessary version of Poetry in contributing.md.
+          poetry-version: 1.3.2
+      - name: Install dependencies
+        run: poetry install --no-root
+      - name: Check that `mkdocs` can build the docs
+        run: poetry run -- mkdocs build --strict
+
   cargo-deny:
     runs-on: ubuntu-latest
     strategy: