Add script to check generated release notes

[thbkrkr]

This adds a script that helps identifying changes that have been labeled as belonging to certain release version where the corresponding commits are not in the release branch. Its main purpose is to discover missing backports.

Testing:

• Success:

> hack/release-notes/check-release-notes.sh v2.7.0
Compare 'merged PR from 2.6.1 to v2.7.0' with 'release-notes/v2.7.0'
✅ LGTM

• Failure:

Simulate an invalid generated release note by deleting an entry:

diff --git a/docs/release-notes/2.7.0.asciidoc b/docs/release-notes/2.7.0.asciidoc
index 1518c41e6..ba3580da8 100644
--- a/docs/release-notes/2.7.0.asciidoc
+++ b/docs/release-notes/2.7.0.asciidoc
@@ -11,7 +11,6 @@
 [float]
 === Enhancements

-* Add a new role for APM Server 8.7.0+ {pull}6605[#6605]
+* Add a new role for APM Server 8.7.0+ {pull}16605[#6605]

> hack/release-notes/check-release-notes.sh v2.7.0
Compare 'merged PR from 2.6.1 to v2.7.0' with 'release-notes/v2.7.0'
❌ Error: no commit found for the following issues:
> #16605

[barkbay]

LGTM! Thanks 🙇

[thbkrkr]

buildkite test this