chore: remove Dependabot config to disable automated dependency PRs (#146)

The .github/dependabot.yml added in #130 immediately opened 15 dependency-
update PRs (#131–#145), which is more churn than wanted. Remove the config
to turn off Dependabot version updates; dependency bumps will be done
manually. (Dependabot security alerts/updates are a separate repo setting,
unaffected by this file.)

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: nicolotognoni