chore(deps): update ghcr.io/open-feature/flagd-testbed docker tag to v0.5.21

Author: renovate[bot]

.github/workflows/ci.yaml

@@ -46,7 +46,7 @@ jobs:
     services:
       # flagd-testbed for flagd-provider e2e tests
       flagd:
-        image: ghcr.io/open-feature/flagd-testbed:v0.5.6
+        image: ghcr.io/open-feature/flagd-testbed:v0.5.21
         ports:
           - 8013:8013
       # sync-testbed for flagd-provider e2e tests