cosmos: changelog update for 0.34.0 release (Azure#4490)

Changelog update for 0.34.0 release. I also tried to backfill some
CHANGELOG PR references we didn't have in the original change.

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Co-authored-by: Fabian Meiswinkel <fabianm@microsoft.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Author: 3 people