What?

Updates the Flex component to satisfy the exhaustive-deps eslint rule.

Why?

Part of the effort in #41166 to apply exhuastive-deps to the Components package

How?

Including rtl.watch() in the dep array can't be checked by ESLint, so we need to extract it into its own variable that can be checked.

Even with that done, we need to disable exhaustive-deps here for now, because we're relying on a value from outside of the useEffect to trigger it, making the dependency invalid. See #41254 (review) for more context.

Testing Instructions

1. From your local Gutenberg directory, run npx eslint --rule 'react-hooks/exhaustive-deps: warn' packages/components/src/flex
2. Confirm that the linter returns no errors
3. Run Storybook locally, confirm the components stories and/or docs still work as expected