Use ID! type instead of String!

[CasperWA]

Description:

Use type ID! instead of String! in the auto-merge workflow for a GitHub GraphQL query.

This PR can be forced through as a proper review makes no sense.
Furthermore, whether it works or not will be made clear in #373.

Fixes #374

Type of change:

• Bug fix.
• New feature.
• Documentation update.

Checklist:

This checklist can be used as a help for the reviewer.

• Is the code easy to read and understand?
• Are comments for humans to read, not computers to disregard?
• Does a new feature has an accompanying new test (in the CI or unit testing schemes)?
• Has the documentation been updated as necessary?
• Does this close the issue?
• Is the change limited to the issue?
• Are errors handled for all outcomes?
• Does the new feature provide new restrictions on dependencies, and if so is this documented?

Comments: