CI: Add path filter to csharp-npgsql project

In this manner, corresponding CI jobs will only be started on changes
within the relevant directories.

Author: amotl

.github/workflows/tests-npgsql.yml

@@ -3,8 +3,12 @@ name: Npgsql Tests
 on:
   push:
     branches: [ main ]
+    paths:
+    - 'by-language/csharp-npgsql/**'
   pull_request:
     branches: [ main ]
+    paths:
+    - 'by-language/csharp-npgsql/**'
   workflow_dispatch:
   schedule:
     - cron: '0 2 * * *'