flow-todo: Switch parser to @babel/eslint-parser, from babel-eslint

Oops, looks like we should have done this in e183358, for zulip#4254.

I'm not sure how much we actually want to keep the `test:flow-todo`
command, but might as well bring it up to date in this way.

We're about to change it again, to hermes-eslint, along with our
other ESLint configs.

tools/flow-todo.eslintrc.yaml

@@ -1,4 +1,4 @@
-parser: babel-eslint
+parser: "@babel/eslint-parser"
 plugins:
   - ft-flow
 rules: