Update HISTORY for 7.3 release (#10031)

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/10031

Reviewed By: riversand963

Differential Revision: D36567741

Pulled By: ltamasi

fbshipit-source-id: 058f8cc856d276db6e1aed07a89ac0b7118c4435

HISTORY.md

 # Rocksdb Change Log
-## Unreleased
+## 7.3.0 (05/20/2022)
 ### Bug Fixes
 * Fixed a bug where manual flush would block forever even though flush options had wait=false.
 * Fixed a bug where RocksDB could corrupt DBs with `avoid_flush_during_recovery == true` by removing valid WALs, leading to `Status::Corruption` with message like "SST file is ahead of WALs" when attempting to reopen.