build(deps): bump DeterminateSystems/update-flake-lock from 20 to 21

Bumps [DeterminateSystems/update-flake-lock](https://github.com/determinatesystems/update-flake-lock) from 20 to 21.
- [Release notes](https://github.com/determinatesystems/update-flake-lock/releases)
- [Commits](https://github.com/determinatesystems/update-flake-lock/compare/v20...v21)

---
updated-dependencies:
- dependency-name: DeterminateSystems/update-flake-lock
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/flake-updates.yml

@@ -17,7 +17,7 @@ jobs:
             experimental-features = nix-command flakes
       - name: Update flake.lock
         id: update
-        uses: DeterminateSystems/update-flake-lock@v20
+        uses: DeterminateSystems/update-flake-lock@v21
       - name: Enable Automerge
         run: gh pr merge --rebase --auto "${{ steps.update.outputs.pull-request-number }}"
         env: