change(deps): Use ECC deps with activation height for NU6

[upbqdn]

Motivation

Zebra 2.0.0 uses ECC deps that don't have NU6 Mainnet activation height set. We must use new versions of these crates that have the activation height set.

Solution

• Unpin ECC deps.
• Bump ECC deps. For reference, see https://github.com/zcash/zcash/blob/a0602ebfb765a965acd8de0b012da28f257a6918/Cargo.toml.
• Enable the legacy-api feature of incrementalmerkletree to be able to serialize note commitment trees using an old serialization format for the z_gettreestate RPC.

Tests

No new tests.

Follow-up Work

• Do a new release.

[arya2]

Looks like the checkpoint job didn't find a cached state: https://github.com/ZcashFoundation/zebra/actions/runs/11559820540/job/32175800783?pr=8978#step:15:3351

Update: I'm running the sync_to_mandatory_checkpoint_mainnet test locally since it seems to timeout just before the mandatory checkpoint height it's trying to reach.

Update: The test passes locally, I think we should admin merge this if it times out again. Increasing the timeout should help otherwise.