docs: remove changelog placeholder note

[kholdrex]

Summary

Remove a leftover maintainer placeholder from the changelog so the published history reads as project documentation rather than template guidance.

Changes

• Delete the trailing placeholder note about documenting older versions.
• Remove the now-unused final separator after the 0.2.0 notes.

Test plan

• RSpec suite passed.
• RuboCop passed with no offenses.
• Gem build completed successfully.
• Diff whitespace check passed.