.github/dependabot.yml: Disable automatic rebasing

Sets the rebase-strategy to "disabled" to prevent automatic
rebasing.

Rebasing can be done manually in the dependabot PR either through
the GitHub UI or the dependabot command.

Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Sean Brogan <sean.brogan@microsoft.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>