release(turborepo): 2.8.21-canary.1 (#12407)
## Release v2.8.21-canary.1
Manual release PR to advance `main` past the partially-published
`2.8.21-canary.1` canary.
The [automated release
run](https://github.com/vercel/turborepo/actions/runs/23352060079/job/67935372332)
failed during npm publish (`@turbo/types` was the only package that
didn't make it). Since the release PR is created after publish, `main`
was never updated to reflect the published version.
This PR applies the same changes the release workflow would have: bumps
`version.txt` and all package versions to `2.8.21-canary.1` so the next
scheduled canary publishes `2.8.21-canary.2`.
