chore: Track the latest v3 contract test release instead of a pinned alpha

The FDv2 contract test job pinned sdk-test-harness to v3.0.0-alpha.6, which
means every harness release needs a manual bump here. In practice the pin went
stale and the job skipped all v3.1.x and v3.2.x tests.

The harness downloader resolves a partial version against the Releases API and
includes prereleases, so "v3" selects the newest v3 release. The FDv1 job in
this repo already floats the same way on the action default of "v2".

Author: jsonbailey

.github/workflows/ci.yml

@@ -67,7 +67,7 @@ jobs:
         with:
           test_service_port: 9000
           token: ${{ secrets.GITHUB_TOKEN }}
-          version: v3.0.0-alpha.6
+          version: v3
           enable_persistence_tests: "true"
 
   windows: