ci: bump lock-threads to v6 to fix token validation failure

[simpleqt]

Problem

The scheduled Lock Threads workflow fails on every run — most recently run 35478525947 (2026-09-20) and each preceding execution.

##[error]"github-token" length must be less than or equal to 100 characters long

v5 of the action validates the github-token input with a 100-character cap, but the automatically-injected GITHUB_TOKEN now exceeds that length, so the action rejects its default input and closed threads are no longer being locked.

Upstream fixed the validation schema in v6.0.2 ("update github-token validation schema", closes dessant/lock-threads#55).

Fix

Bump dessant/lock-threads v5 → v6. The inputs used in this workflow are unchanged in v6, so lock behaviour stays the same once the workflow runs again.

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