Respect the start and end sha inputs

[timflannagan]

The logic that filters out merged PRs that fall outside of the start..end sha commit range is currently unimplemented. See https://github.com/kgateway-dev/changelog-generator/blob/main/internal/generator/generator.go#L71-L74 and https://github.com/kgateway-dev/changelog-generator/blob/main/internal/generator/generator.go#L77.

[timflannagan]

Hmm, this seems slightly awkward to implement for patch releases as the GH search API doesn't allow me to configure the branch.