Make release-notes comment step non-fatal (fixes check_release_notes 403 on bot PRs)

[T-Gro]

Problem

The required check_release_notes check intermittently fails and blocks PRs (e.g. the Maestro/darc dependency update #20133) even when release-notes validation itself passes with ""No release notes required"".

check_release_notes runs via pull_request_target, so GitHub executes the workflow from the default branch (main) for every PR regardless of its base branch. After the validation succeeds, the Create or update comment step tries to post the informational status comment and fails:

HttpError: Resource not accessible by integration
POST /repos/dotnet/fsharp/issues/<n>/comments  -> 403

Observed behaviour across recent runs: when a bot status comment already exists the step uses updateComment and succeeds, but on a PR with no existing comment the createComment call returns 403 and fails the whole job. The runner is granted Issues: write, so this is an environment/lockdown restriction on the Actions token creating new issue comments — not something the workflow permissions: block can fix. This turns a passing validation into a red required check and blocks auto-merge.

Fix

Mark the Create or update comment step with continue-on-error: true. Posting the informational comment is best-effort and must never fail the check. The real gate — exit 1 when release notes are genuinely missing — is unchanged.

Because pull_request_target always runs the default-branch workflow, this single change to main fixes check_release_notes for all PRs, including #20133.