fix(feature-activation): add signal bits to minimally valid block

[glevco]

Motivation

The BlockTemplate.generate_minimally_valid_block() method is not setting signal bits as it should.

Acceptance Criteria

• Add signal bits to minimally valid block template

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (68d1edc) 84.74% compared to head (fa6efd5) 84.67%.
Report is 11 commits behind head on master.

❗ Current head fa6efd5 differs from pull request most recent head 1360614. Consider uploading reports for the commit 1360614 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #774      +/-   ##
==========================================
- Coverage   84.74%   84.67%   -0.08%     
==========================================
  Files         259      259              
  Lines       21939    21931       -8     
  Branches     2964     2964              
==========================================
- Hits        18593    18570      -23     
- Misses       2703     2712       +9     
- Partials      643      649       +6     

Files	Coverage Δ
hathor/mining/block_template.py	94.33% <100.00%> (ø)

... and 27 files with indirect coverage changes

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