Prepare testnet 3.39

[beqaabu]

• update tx payment pallet LengthToFee TransactionByteFee

• fix clippy

• cargo toml to v 39

Description of proposed changes

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Practical tips for PR review & merge:

• All GitHub Actions report PASS
• Newly added code/functions have unit tests
  • Coverage tools report all newly added lines as covered
  • The positive scenario is exercised
  • Negative scenarios are exercised, e.g. assert on all possible errors
  • Assert on events triggered if applicable
  • Assert on changes made to storage if applicable
• Modified behavior/functions - try to make sure above test items are covered
• Integration tests are added if applicable/needed

[atodorov]

https://github.com/gluwa/creditcoin3/actions/runs/11897088100/job/33150510675?pr=527 is false positive, same explanation as in #525 (comment) b/c it is the same changes that are brought up in this PR.

Note: arguably the wording in the check script is chosen poorly. Should be something like " MILLISECS_PER_BLOCK detected in change request!"

[github-actions]

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