CI/CD: install .NET 6 (matches binding TFM after LOC-6563)

PR #57 dropped the binding's <TargetFramework> from net7.0 to net6.0
(commit d968e36) so the SDK could keep its .NET 6 consumers. The
GitHub Actions workflows still install `dotnet-version: 7.0.410`,
which means `dotnet test BrowserStackLocalIntegrationTests` can't
launch the net6.0 test host — failing with:

  Framework: 'Microsoft.NETCore.App', version '6.0.0' (x64)
  The following frameworks were found:
    7.0.20, 8.0.6, 8.0.22, 8.0.27, 9.0.6, 9.0.16, 10.0.8

Switch setup-dotnet@v3 to install 6.0.x in both ci.yml and cd.yml
so the integration test host can find the .NET 6 runtime it was
built against.

CI workflow run that surfaced this:
https://github.com/browserstack/browserstack-local-csharp/actions

Tracks LOC-6563.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: yashdsaraf

.github/workflows/cd.yml

@@ -25,7 +25,7 @@ jobs:
     - name: Setup .NET Core
       uses: actions/setup-dotnet@v3
       with:
-        dotnet-version: 7.0.410
+        dotnet-version: 6.0.x
     - name: Run Integration Tests
       env:
         BROWSERSTACK_USERNAME: ${{ secrets.BROWSERSTACK_USERNAME }}

.github/workflows/ci.yml

@@ -25,7 +25,7 @@ jobs:
     - name: Setup .NET Core
       uses: actions/setup-dotnet@v3
       with:
-        dotnet-version: 7.0.410
+        dotnet-version: 6.0.x
     - name: Run Integration Tests
       env:
         BROWSERSTACK_USERNAME: ${{ secrets.BROWSERSTACK_USERNAME }}