ci: prefix dependabot commits with chore

[eluttine]

Dependabot writes commits as Bump x from y to z, which has no conventional-commit prefix, so the Block unconventional commits workflow fails on every dependabot PR. Seven are open and red for this reason alone — the updates themselves are fine.

Setting commit-message.prefix makes dependabot write chore: bump x from y to z instead.

Fixing it here rather than in the workflow on purpose: block-unconventional-commits.yml comes from the nextcloud/.github org template, so excluding the bot there would have to be re-applied on every template update.

Existing PRs keep their old commits — they need @dependabot rebase to pick up the new message format.