fix: Bump dessant/lock-threads to v6

[mattwang44]

Description

(During my investigation to why v8.8.1 is not released to the tf registry, though not related) I noticed the scheduled Lock Threads workflow has been failing on every run (e.g. run #27857912049):

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

dessant/lock-threads@v5 validates github-token with a Joi.string().trim().max(100) schema. GitHub's GITHUB_TOKEN now exceeds 100 characters, so the input fails validation and the job errors out before doing anything.

dessant/lock-threads@v6 raises that cap to max(1000), which resolves the failure. It also moves the action runtime to node24, clearing the Node.js 20 is deprecated warning that the same run emits.

This bumps the action v5 → v6. No input changes are required — all inputs used by this workflow are unchanged in v6.

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I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues. If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.