fix: replaced hard-coded MaxLookaheadNonce with a default lookback factor

[ws4charlie]

Description

1. The hard-coded MaxLookaheadNonce 120 works for Ethereum but won't make sense for other EVM chains. The PR replaced the MaxLookaheadNonce with default lookback factor.
2. The newly calculated number will be same as the existing number used for Ethereum mainnet.

Closes: 1484

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Include instructions and any relevant details so others can reproduce.

• Tested CCTX in localnet
• Tested in development environment
• Go unit tests
• Go integration tests
• Tested via GitHub Actions

Checklist:

• I have added unit tests that prove my fix feature works

[github-actions]

!!!WARNING!!!
nosec detected in the following files: zetaclient/zetacore_observer.go

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 69.96%. Comparing base (379a056) to head (93f74c0).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2033      +/-   ##
===========================================
- Coverage    69.96%   69.96%   -0.01%     
===========================================
  Files          249      249              
  Lines        14039    14040       +1     
===========================================
  Hits          9823     9823              
- Misses        3758     3759       +1     
  Partials       458      458              

Files	Coverage Δ
zetaclient/zetacore_observer.go	16.58% <0.00%> (-0.08%)	⬇️

[skosito]

some comments about name changes, but looks good