chore: Post v6.0.0 release maintenance

[spoo-bar]

Closes: #510

• Cleanup the changelog to match with the v6 release
• Rename the upgrade handlers from v5=>v6
• Update the initial version name in interchaintest to v6
• Added changelog entry for 4.0.3

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (83424ff) 0.00% compared to head (60a44ba) 64.62%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #540 +/- ## ========================================= + Coverage 0 64.62% +64.62% ========================================= Files 0 88 +88 Lines 0 4851 +4851 ========================================= + Hits 0 3135 +3135 - Misses 0 1550 +1550 - Partials 0 166 +166 ```

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