Compute block hash correctly for capella payload

[avalonche]

📝 Summary

⛱ Motivation and Context

📚 References

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✅ I have run these commands

• make lint
• make test-race
• go mod tidy

[codecov-commenter]

Codecov Report

Merging #457 (79f8c25) into main (256be1b) will increase coverage by 0.70%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main     #457      +/-   ##
==========================================
+ Coverage   66.66%   67.37%   +0.70%     
==========================================
  Files           8        8              
  Lines        1212     1226      +14     
==========================================
+ Hits          808      826      +18     
+ Misses        355      351       -4     
  Partials       49       49              

Flag	Coverage Δ
unittests	67.37% <100.00%> (+0.70%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
server/utils.go	80.95% <100.00%> (+5.95%)	⬆️

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