fix: add flush=true to preStop hook

[alextricity25]

What this PR does / why we need it:
WAL needs to be flushed per https://github.com/grafana/loki/blob/main/docs/sources/operations/storage/wal.md#how-to-scale-updown

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
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