Npgsql: Add .NET 8 framework to test matrix

Author: amotl

.github/workflows/test-npgsql.yml

@@ -42,7 +42,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ 'ubuntu-latest' ]
-        dotnet-version: [ '5.0.x', '6.0.x', '7.0.x' ]
+        dotnet-version: [ '5.0.x', '6.0.x', '7.0.x', '8.0.x' ]
         npgsql-version: [ '6.0.10', '7.0.6' ]
         cratedb-version: [ 'nightly' ]