Update Godot.gitignore to ignore export_credentials.cfg

[sdepouw]

As of May 2023, export_presets.cfg no longer contains sensitive information in Godot. Sensitive information now resides in export_credentials.cfg

Reasons for making this change:

When starting to use Godot, I searched for .gitignore files and found this repository. I saw that it was out of date compared to changes made to Godot that are nearly two years old as of the time of making this PR.

Links to documentation supporting these rule changes:

This Godot Pull Request shows the change being made to no longer store sensitive information in export_presets.cfg, and instead place them in export_crednetials.cfg. Here is the completed proposal. Quoting the proposal:

Godot stores export settings including which resources to bundle, app icons, export capabilities in the export_presets.cfg. These settings are a core part of development, and we normally want to include these in source control. Unfortunately Godot also stores the keystore credentials in the same export file. This means we either have to commit credentials to our repo, or we have to manually recreate the presets between developers, and somehow sync changes. Both are really bad options. [...] It would be better to have keystore credentials in their own config file alongside exports_presets.cfg. Something like exports_credentials.cfg or similar.