chore(ci): remove per-repo Dependabot alert workflows (#4384)

## ✅ Checklist

- [x] I have followed every step in the [contributing
guide](https://github.com/triggerdotdev/trigger.dev/blob/main/CONTRIBUTING.md)
- [x] The PR title follows the convention.
- [ ] I ran and tested the code works — n/a, this PR only deletes two
workflow files

---

## Summary

**Before:** two scheduled workflows in this repo posted Dependabot
digests to Slack — a critical-alert check every morning at 08:00 UTC,
and a summary of all open alerts on Mondays at 08:00 UTC.

**After:** neither runs. This reporting is handled centrally now, so the
two in-repo workflows were duplicating it.

**How:** deletes `.github/workflows/dependabot-critical-alerts.yml` and
`.github/workflows/dependabot-weekly-summary.yml`. Both were
self-contained — inline shell, no shared scripts or composite actions —
so nothing else in `.github/` referenced them.

Dependabot itself is unchanged: `.github/dependabot.yml`, alerts, and
version updates all keep working. This removes only the two Slack
notifiers.

The `ENABLE_DEPENDABOT_ALERTS` repository variable existed only to
switch these two workflows off. Nothing else reads it, so it can be
removed from the repository settings if it's set.

---

## Testing

No runtime code changes — this PR only removes two scheduled workflow
files. Verified that nothing else in the repo references either
filename, either workflow name, or the `ENABLE_DEPENDABOT_ALERTS`
variable.

---

## Changelog

Removed the two in-repo scheduled workflows that posted Dependabot
digests to Slack.

Co-authored-by: Claude <noreply@anthropic.com>

Author: claude[bot]