chore(ci): add script for checking for unreleased changes on release branches

Checklist

[x] Change(s) are motivated and described in the PR description
[x] Testing strategy is described if automated tests are not included in the PR
[x] Risks are described (performance impact, potential for breakage, maintainability)
[x] Change is maintainable (easy to change, telemetry, documentation)
[x] Library release note guidelines are followed or label changelog/no-changelog is set
[x] Documentation is included (in-code, generated user docs, public corp docs)
[x] Backport labels are set (if applicable)
[x] If this PR changes the public interface, I've notified @DataDog/apm-tees.

[x] Title is accurate
[x] All changes are related to the pull request's stated goal
[x] Description motivates each change
[x] Avoids breaking API changes
[x] Testing strategy adequately addresses listed risks
[x] Change is maintainable (easy to change, telemetry, documentation)
[x] Release note makes sense to a user of the library
[x] Author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
[x] Backport labels are set in a manner that is consistent with the release branch maintenance policy