Serialize integration test runs to avoid rate limiting

Add a concurrency group to the IntegrationTests job so only one run
executes at a time across PRs/branches. Concurrent runs shared one
Dropbox account and tripped API rate limits.

Author: AndreyVMarkelov

.github/workflows/ci.yml

@@ -32,6 +32,9 @@ jobs:
           fail_ci_if_error: true
           directory: dropbox-sdk-dotnet/Dropbox.Api.Unit.Tests/TestResults/
   IntegrationTests:
+    concurrency:
+      group: integration-tests
+      cancel-in-progress: false
     strategy:
       matrix:
         os: [macos-latest, ubuntu-latest, windows-latest]