ci: bump Node to 20 for css-minimizer-webpack-plugin@8 engine requirement

css-minimizer-webpack-plugin@8 and serialize-javascript@7 require Node
>= 20. Bumps sphinx-theme-builder's nodeenv pin from 18.18.0 to 20.18.0
and the matching setup-node version across CI/docs/update-snapshots
workflows. Without this, the editable install fails with
'ReferenceError: crypto is not defined' inside serialize-javascript.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: mmcky

.github/workflows/ci.yml

@@ -43,7 +43,7 @@ jobs:
       - name: Setup Node.js
         uses: actions/setup-node@v6
         with:
-          node-version: '18'
+          node-version: '20'
           cache: 'npm'
       - name: Install Playwright
         run: |

.github/workflows/docs.yml

@@ -34,7 +34,7 @@ jobs:
       - name: Set up Node.js
         uses: actions/setup-node@v6
         with:
-          node-version: "18"
+          node-version: "20"
           cache: "npm"
 
       - name: Install Node.js dependencies and build assets

.github/workflows/update-snapshots.yml

@@ -62,7 +62,7 @@ jobs:
       - name: Setup Node.js
         uses: actions/setup-node@v6
         with:
-          node-version: '18'
+          node-version: '20'
           cache: 'npm'
 
       - name: Install Playwright
@@ -169,7 +169,7 @@ jobs:
       - name: Setup Node.js
         uses: actions/setup-node@v6
         with:
-          node-version: '18'
+          node-version: '20'
           cache: 'npm'
 
       - name: Install Playwright

pyproject.toml

@@ -3,7 +3,7 @@ requires = ["sphinx-theme-builder >= 0.2.0a7"]
 build-backend = "sphinx_theme_builder"
 
 [tool.sphinx-theme-builder]
-node-version = "18.18.0"
+node-version = "20.18.0"
 theme-name = "quantecon_book_theme"
 additional-compiled-static-assets = [
   "locales/"