Don't run message length validation on `SDK_CHANGELOG.next.json` (#2940)

The new changelog message length validation is running when attempting
to release the SDK, which reads historical entries from
`SDK_CHANGELOG.next.json`, and these older entries didn't have a length
validation. Thus, this validation can't run during render time.

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._