chore: remove deprecated dotnet6 runtime templates

related to  SAM CLI (aws/aws-sam-cli#8914). Remove all dotnet6 templates, manifest
entries, tests, and CI references.

Author: sliedig

.github/workflows/build_test_invoke.yml

@@ -357,12 +357,6 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - version: '6'
-            type: 'Test'
-            file: 'tests/integration/unit_test/test_unit_test_dotnet6.py'
-          - version: '6'
-            type: 'Invoke'
-            file: 'tests/integration/build_invoke/dotnet/test_build_invoke_dotnet6.py'
           - version: '8'
             type: 'Test'
             file: 'tests/integration/unit_test/test_unit_test_dotnet8.py'