[MBL-1780] Remove deleted method from our beta changelog

[ifosli]

📲 What

Update beta-changelog.rb to using File.exist instead of File.exists, as the latter no longer exists in Ruby 3.3.

🤔 Why

I think this will unblock the beta build getting uploaded. The full Ruby error also includes a warning to aws-sdk-core about a change that's happening in Ruby 3.4, but I'm ignoring this for now. I think the fix for that warning is to update our aws gem versions, but I'd rather not do that without understanding what they're doing so I can test the changes properly, and I don't expect a ruby warning to break the script.

👀 See

Jira

✅ Acceptance criteria

• Tests pass
• I'm not testing that the beta build works before merging this pr, since I'm not sure how. (I tried creating a pr in the private repo instead, but it looks like the beta build only gets created after submit.) Plus, this pr is definitely part of the solution so I see no harm in confirming if it's the entire solution after it's submitted.