Remove Dependabot configuration (#1128)

As part of #incident-51602-public, we are temporarily disabling all
automated dependency updaters to reduce exposure to potential zero-day
vulnerabilities in recent releases.

This PR removes the Dependabot/Renovate configuration from this
repository until further notice.

⚠️ Do not merge if your repository is managed by ADMS.

Please refer to #incident-51602-public for updates and to confirm when
it is safe to re-enable.

Author: chouetz