Sort the changelog properly

Summary:
While working on the changelog, I realized that version 0.75.4 ended up in between RCs of 0.76

To avoid to lose the changelog of 0.75.4 once we release 0.76, I moved it to the right place, above 0.75.3's changelog
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## Changelog:
[Internal] - Sort Changelog properly

Reviewed By: cortinico

Differential Revision: D64470420

fbshipit-source-id: 0602b6cf4d7b9d9b3befb2535de6079d28545a15