chore(deps): update dependency libs/shared/flagd-core/test-harness to v0.5.21 (#1154)

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

Author: renovate[bot]

.gitmodules

@@ -13,7 +13,7 @@
 [submodule "libs/shared/flagd-core/test-harness"]
 	path = libs/shared/flagd-core/test-harness
 	url = https://github.com/open-feature/flagd-testbed
-	branch = v0.5.20
+	branch = v0.5.21
 [submodule "libs/shared/flagd-core/spec"]
 	path = libs/shared/flagd-core/spec
 	url = https://github.com/open-feature/spec