Use get_prs_between_tags for release notes once release-toolkit ships fail_on_error #591

fastlane/Fastfile builds the GitHub Release body by calling
Fastlane::Helper::GithubHelper#generate_release_notes directly, rather than the
get_prs_between_tags action that wraps it.

That was a deliberate workaround: the action rescued every StandardError and
returned the message as the changelog, so a transient API failure would publish a
release whose notes read ❌ Error computing the list of PRs…. We push the tag
before creating the Release, so that needs to fail the step instead.

wordpress-mobile/release-toolkit#772 added a fail_on_error: parameter and is
merged, but not yet in a released gem — we're pinned to ~> 14.11, locked at
14.11.3, and the entry is still under ## Trunk in the toolkit CHANGELOG.

Once it ships:

• Bump fastlane-plugin-wpmreleasetoolkit in Gemfile.lock to a version
  containing task: sign the demo apps into self-hosted sites listed in ~/.config/wp/sites #772
• Replace the generated_release_notes helper's GithubHelper call with
  get_prs_between_tags(..., fail_on_error: true)
• Drop the now-stale comment explaining why we bypass the action

fail_on_error: true is required, not optional. Without it the action swallows the
400 Invalid previous_tag parameter that a malformed previous_tag produces and
publishes the error text as the release body — the exact failure #772 exists to
prevent.

Context: #588 (comment)