fix: use correct config key for db_backend (backport #17406)

[mergify]

This is an automatic backport of pull request #17406 done by Mergify.
Cherry-pick of 6b0f8ad has failed:

On branch mergify/bp/release/v0.46.x/pr-17406
Your branch is up to date with 'origin/release/v0.46.x'.

You are currently cherry-picking commit 6b0f8add4.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   CHANGELOG.md
	both modified:   server/config/toml.go
	both modified:   server/util.go
	both modified:   server/util_test.go
	deleted by us:   tools/confix/data/v0.47-app.toml
	deleted by us:   tools/confix/data/v0.50-app.toml

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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