ci: bump lock-threads to v6 so the scheduled lock job can run

[tushardev-365]

The scheduled lock job fails on every run before locking anything. The last 10 runs of lock.yml are all failures, each with:

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

(from run 32324081534; the earlier runs fail identically)

The workflow pins dessant/lock-threads@v4, and that version validates its github-token input with a 100 character cap in its Joi schema:

'github-token': Joi.string().trim().max(100),

The Actions token has grown past 100 characters, so validation rejects it and the action exits before doing any work. The cap raise landed in v6.0.2 specifically: v4, v5.0.x, v6.0.0 and v6.0.1 all still validate max(100), and v6.0.2 moved it to 1000. So v6.0.2 is the first safe pin, and it is what the moving v6 tag currently points at.

Worth noting since it is the first thing one might try: this workflow does not pass github-token at all, but removing or not passing the input does not avoid the bug, because the action declares default: ${{ github.token }} in its action.yml and the default goes through the same validation.

I checked every input this workflow passes against the v6.0.2 schema and all of them exist there unchanged, including their accepted values, so the pin bump is the entire fix. The new pin is the commit the v6.0.2 release tag resolves to, keeping the SHA-pinned style.

I could not run this against the repo since the workflow only triggers on schedule, but the same one-line bump fixed the identical failure elsewhere, and the v6 cohort of repos running this action shows green runs.