Raise if path isn't a string

What does this MR do and why?

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/371994

Kicking off a quick MR to see if specs fail. I will clean it up with specs depending on how much stuff breaks (if at all!)

Screenshots or screen recordings

N/A

How to set up and validate locally

From the rails console

[1] pry(main)> attack = [ ".." , "..", "..", "..", "..", "..", "..", "..", "..", "..", "etc", "passwd"]
=> ["..", "..", "..", "..", "..", "..", "..", "..", "..", "..", "etc", "passwd"]
[2] pry(main)> Gitlab::Utils.check_path_traversal! attack
Gitlab::Utils::PathTraversalAttackError: Invalid path
from /home/dcouture/gdk/canonical/gitlab/lib/gitlab/utils.rb:20:in `check_path_traversal!'

Previously this did not raise

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