Revert "Update dependency puma to '~> 6.4'"

What
---
This reverts commit 388913eb.

Why
---
We started seeing 500 errors when we don't have a URL encoding query
string:

```shell
$ curl -I 'http://gdk.test:3000/-/merge_requests?sort=created_date&state=<th:t=\"%24{dfb}%23foreach

'
HTTP/1.1 400 Bad Request
Date: Mon, 25 Sep 2023 14:41:36 GMT
```

Signed-off-by: Steve Xuereb <sxuereb@gitlab.com>