Make nu6 rust feature stable

[oxarbitrage]

We want to add the testnet network activation to Zebra but we need the same in zcash_primitives. We also need to remove the zcash_unstable = "nu6" feature.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 60.56%. Comparing base (104c0c2) to head (cd56afc).
Report is 2 commits behind head on main.

Files	Patch %	Lines
components/zcash_protocol/src/consensus.rs	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1454      +/-   ##
==========================================
- Coverage   60.76%   60.56%   -0.21%     
==========================================
  Files         138      139       +1     
  Lines       16313    16372      +59     
==========================================
+ Hits         9912     9915       +3     
- Misses       6401     6457      +56     

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[oxarbitrage]

• Need to remove Testnet height and leave it to None for now.

[daira]

utACK

[str4d]

utACK. Changelog was not updated, but I can do that in a separate PR.