fix(deps): update module cloud.google.com/go/firestore to v1.20.0

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
cloud.google.com/go/firestore	v1.18.0 -> v1.20.0

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Configuration

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🚦 Automerge: Enabled.

♻ Rebasing: Whenever PR is behind base branch, or you tick the rebase/retry checkbox.

🔕 Ignore: Close this PR and you won't be reminded about this update again.

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• If you want to rebase/retry this PR, check this box

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[renovate]

⚠️ Artifact update problem

Renovate failed to update an artifact related to this branch. You probably do not want to merge this PR as-is.

♻ Renovate will retry this branch, including artifacts, only when one of the following happens:

• any of the package files in this branch needs updating, or
• the branch becomes conflicted, or
• you click the rebase/retry checkbox if found above, or
• you rename this PR's title to start with "rebase!" to trigger it manually

The artifact failure details are included below:

File name: backend/go.sum

Command failed: go get -t ./...
go: module cloud.google.com/go/[email protected] requires go >= 1.24.0; switching to go1.24.10
go: downloading go1.24.10 (linux/amd64)
go: download go1.24.10: golang.org/[email protected]: verifying module: checksum database disabled by GOSUMDB=off