Use newer plugin installation syntax

Author: SimonCW

.github/workflows/build-docs.yml

@@ -43,7 +43,7 @@ jobs:
         run: |
           python -m pip install --upgrade pip
           python -m pip install "poetry==1.5.1"
-          python -m poetry plugin add poetry-version-plugin
+          python -m poetry self add poetry-version-plugin
       - name: Configure poetry
         run: python -m poetry config virtualenvs.create false
       - name: Install Dependencies