chore(deps): update ghcr.io/open-feature/flagd-testbed-unstable docker tag to v0.5.21 (#1159)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

libs/providers/flagd/docker-compose.yaml

@@ -4,7 +4,7 @@ services:
     ports:
       - 8013:8013
   flagd-unstable:
-    image: ghcr.io/open-feature/flagd-testbed-unstable:v0.5.20
+    image: ghcr.io/open-feature/flagd-testbed-unstable:v0.5.21
     ports:
       - 8014:8013
   flagd-sync: