add 'make go-mod-tidy' to serially run tidy on all submodules in the correct order (#7179)

- also make go-mod-tidy a dependency of update-vendor

GNUmakefile

@@ -219,7 +219,13 @@ test: other-consul dev-build vet test-install-deps test-internal
 test-install-deps:
 	go test -tags '$(GOTAGS)' -i $(GOTEST_PKGS)
 
-update-vendor:
+go-mod-tidy:
+	@echo "--> Running go mod tidy"
+	@cd sdk && go mod tidy
+	@cd api && go mod tidy
+	@go mod tidy
+
+update-vendor: go-mod-tidy
 	@echo "--> Running go mod vendor"
 	@go mod vendor
 	@echo "--> Removing vendoring of our own nested modules"