fix(ci): don't fail open when EC release-notes dispatch fails

[kriscoleman]

The generate-ec-release-notes-pr step already checks the HTTP status of the repository_dispatch to replicated-docs and exit 1s on a non-2xx response. But continue-on-error: true masked that failure: GitHub reports the step conclusion as success, so a 404 (e.g. an under-privileged GH_PAT) still passed green — the exact fail-open this was meant to catch.

Removing continue-on-error: true lets the 404 fail the step and surface as a real, red failure. This job runs last (needs: [release, release-app, get-tag]), so the product release is already complete when it runs — a failure here only flags that the docs release-notes dispatch failed, it does not undo or block the release itself.

Verified against run 31411065177: the dispatch returned 404, the step exit 1d, but the step conclusion was success because of continue-on-error.

Note: this makes the failure visible; the dispatch itself still 404s until the GH_PAT secret holds a token with write (Contents: write) access to replicated-docs (tracked separately).