Prepare changelog for 1.8

[awaelchli]

What does this PR do?

Cleans up the changelog sections in preparation for the 1.8 release.
Version numbers and dates will be updated in another PR.

Before submitting

• Was this discussed/approved via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

PR review

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Did you have fun?

I made sure I had fun coding 🙃

cc @Borda @rohitgr7 @Felonious-Spellfire

[tchaton]

LGTM !

[codecov]

Codecov Report

Merging #15111 (3704bdf) into master (672b5cb) will increase coverage by 1%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #15111     +/-   ##
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+ Coverage      82%      84%     +1%     
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  Files         408      337     -71     
  Lines       29877    26296   -3581     
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- Hits        24571    21973   -2598     
+ Misses       5306     4323    -983