CI: Synchronize Python versions between "on-pr" vs. "nightly" jobs

Author: amotl

.github/workflows/nightly.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       matrix:
         os: ['ubuntu-22.04']
-        python-version: ['3.7', '3.8', '3.9', '3.10', '3.11', '3.12', '3.13']
+        python-version: ['3.9', '3.10', '3.11', '3.12', '3.13', '3.14']
         cratedb-version: ['nightly']
 
       fail-fast: false