Update dependabot groups to open PRs for ECC dependency updates separately

[arya2]

Motivation

Zebra's ECC dependencies usually have some breaking changes, or in some cases (for zcash_proofs/zcash_primitives) the updates need to be coordinated with zcash_script updates, so we usually need to split them out of prod dependency update PRs.

It would be easier to define them as their own dependency update group so they won't need to be split out manually.