docs: update changelog (#346)

[egonspace]

Description

Right relation of CHANGELOG and RELEASE_CHANGELOG

• main branch before making rc branch
  • CHANGELOG: include only Unreleased set
  • RELEASE_CHANGELOG: include all vx.x.x sets
• when creating rc branch
  • move all items of Unreleased set to RELEASED_CHANGELOG
  • CHANGELOG: empty Unreleased set
  • RELEASE_CHANGELOG: new vx.x.x set and existing all vx.x.x sets
• main branch after merging rc branch to main
  • starting with empty CHANGELOG
  • having updated RELEASE_CHANGELOG

But for now, rc0/v0.43.0 doesn't have empty CHANGELOG. It's my mistake.
After merging this PR to main, I'll update CHANGELOG of main branch.

closes: #XXXX

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• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer
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