fix(core-api): return transaction timestamp instead of block timestamp

[spkjp]

Summary

Always return the transaction timestamp instead of preferring the block timestamp so that a transaction can be properly verified. Only changed the v2 endpoint, since the v1 endpoint is deprecated anyway.

What kind of change does this PR introduce?

• Bugfix
• New feature
• Refactoring / Performance Improvements
• Build-related changes
• Documentation
• Tests / Continuous Integration
• Other, please describe:

Does this PR introduce a breaking change?

• Yes
• No

Does this PR release a new version?

• Yes
  • All tests are passing
  • All benchmarks are passing without any major regressions
  • Sync from 0 works on mainnet
  • Sync from 0 works on devnet
  • Starting a new network and forging on it work
  • Explorer is fully functional
  • Wallets are fully functional
• No

Checklist

• I have read the CONTRIBUTING documentation
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[codecov-io]

Codecov Report

Merging #2513 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2513      +/-   ##
==========================================
- Coverage   66.02%   66.01%   -0.01%     
==========================================
  Files         400      400              
  Lines        8617     8616       -1     
  Branches      437      395      -42     
==========================================
- Hits         5689     5688       -1     
- Misses       2883     2884       +1     
+ Partials       45       44       -1

Impacted Files	Coverage Δ
...ore-api/src/versions/2/transactions/transformer.ts	100% <ø> (ø)	⬆️
packages/core-logger-winston/src/formatter.ts	42.85% <0%> (ø)	⬆️

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