Modify regex for Releases API filepath

What does this MR do?

Resolving tech debt from #208289 (closed)

We are exposing filepath_url but I couldn't find the code where we're exposing filepath 🔍 🤔

No, and it since was renamed to direct_asset_url, and we've forgotten to remove filepath from the schema. So removing it

> * Prefer `%r{}` if the regexp contains slashes so we don't have to escape them
> * Add `(?: ... )` if we don't need to capture groups which is here the case I believe
> 
> 
> ```suggestion:-0+0
>     FILEPATH_REGEX = %r{\A/(?:[\-\.\w]+/?)*[\da-zA-Z]+\z}.freeze
> ```

☝ Done

> * Do we want to prevent directory traversal by disallowing `/../../../../../etc/passwd`? :thinking: 

This path won't be used to locate files on disc, it will be resolved as part of the URL, so there nothing to traverse right now.

> * Do we allow unicode in file path? If not, why not?

We can add it later if we realize that we need to.

> * Do we have access to these filepath? If so, could we just check its existence instead of checking via regex? :thinking: 

No, it's not a filepath on FS, is just something in /group/project/releases/my-release/assets/{filepath} and GitLab will redirect to Link.url.

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