ci: bump checkout and setup-python to Node 24 (v6) actions

Node 20 actions are deprecated and force-migrated to Node 24 on 2026-06-16.
Bump actions/checkout@v4 -> v6 and actions/setup-python@v5 -> v6 in the test
and release workflows. Inputs are unchanged; only the runtime updates.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: pavelkomarov

.github/workflows/release.yml

@@ -8,8 +8,8 @@ jobs:
       TWINE_USERNAME: __token__
       TWINE_PASSWORD: ${{ secrets.PYPI_TOKEN }}
     steps:
-    - uses: actions/checkout@v4
-    - uses: actions/setup-python@v5
+    - uses: actions/checkout@v6
+    - uses: actions/setup-python@v6
       with:
         python-version: '3.x'
     - run: |

.github/workflows/test.yml

@@ -8,10 +8,10 @@ jobs:
   run-tests:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 0 # important for Coveralls to read commit history
-      - uses: actions/setup-python@v5
+      - uses: actions/setup-python@v6
         with:
           python-version: '3.13' # because cvxopt binaries for 3.14 don't yet exist, and building is terrible
       - name: tests and coverage