docs: Update backup script to match manual/automatic backups

[Quantum-Cucumber]

Description

Immich now uses pg_dump instead of pg_dumpall per #23978.

The web interface does not accept the backups produced by pg_dumpall (#26421)

This updates the backup script template to use pg_dump - matching the admin backup docs and the automatic backup process

Fixes #26421

How Has This Been Tested?

Changes match the commands described in the backup and restore admin guide

Checklist:

• I have carefully read CONTRIBUTING.md
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation if applicable
• I have no unrelated changes in the PR.
• I have confirmed that any new dependencies are strictly necessary.
• I have written tests for new code (if applicable)
• I have followed naming conventions/patterns in the surrounding code
• All code in src/services/ uses repositories implementations for database calls, filesystem operations, etc.
• All code in src/repositories/ is pretty basic/simple and does not have any immich specific logic (that belongs in src/services/)

Please describe to which degree, if any, an LLM was used in creating this pull request.

None
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[immich-push-o-matic]

📖 Documentation deployed to docs.pr-26810.preview.immich.app