Remove signal reason unable_to_verify_signature and replace with signature_rejected

[jordandcarter]

Description

Replaces an unsupported signal reason unable_to_verify_signature with signature_rejected to keep the number of signal reasons small.

Signal reason unable_to_verify_signature was added in #2945 but wasn't added to the rails app - this lead to builds getting stuck running. This has now been fixed in the rails app by handling unknown signal reasons but during that investigation I found existing discussion around additional signal reasons which argues not to add more signal reasons. To discourage more signal reasons, and to avoid not adding them to the app I've added some code comments.

Context

Escalation
Linear

Testing

• Tests have run locally (with go test ./...). Buildkite employees may check this if the pipeline has run automatically.
• Code is formatted (with go fmt ./...)

[DrJosh9000]

Love some thorough commenting ✨