.NET: Stop testing Npgsql EF Core 10 with net8.0 and net9.0

Package Npgsql.EntityFrameworkCore.PostgreSQL 10.0.0 is not compatible
with {net8.0,net9.0}.

Author: amotl

.github/workflows/lang-csharp-efcore.yml

@@ -49,10 +49,13 @@ jobs:
         npgsql-version: [
           '8.*',
           '9.*',
-          '10.*',
-          'prerelease',
         ]
         cratedb-version: [ 'nightly' ]
+        include:
+          - dotnet-version: '10.x'
+            npgsql-version: '10.*'
+          - dotnet-version: '10.x'
+            npgsql-version: 'prerelease'
 
     # https://docs.github.com/en/free-pro-team@latest/actions/guides/about-service-containers
     services: