promtail update to 6.15.3-bb.3

General MR

Summary

(Summarize the purpose of the MR)

Relevant logs/screenshots

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Linked Issue

Closes big-bang/product/packages/promtail#34 (closed)

big-bang/product/packages/promtail!55 (merged)

Upgrade Notices

(Include any relevant notes about upgrades here or write "N/A" if there are none)