Update release version to 6.20 (#8199)

Summary:
Update release version to 6.20

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8199

Test Plan: No code change

Reviewed By: ajkr

Differential Revision: D27838750

Pulled By: akankshamahajan15

fbshipit-source-id: f02f722fc6bdd37d626d47a0e932bbecea3507a8

HISTORY.md

@@ -1,5 +1,5 @@
 # Rocksdb Change Log
-## Unreleased
+## 6.20.0 (04/16/2021)
 ### Behavior Changes
 * `ColumnFamilyOptions::sample_for_compression` now takes effect for creation of all block-based tables. Previously it only took effect for block-based tables created by flush.
 * `CompactFiles()` can no longer compact files from lower level to up level, which has the risk to corrupt DB (details: #8063). The validation is also added to all compactions.

include/rocksdb/version.h

@@ -10,7 +10,7 @@
 #include "rocksdb/rocksdb_namespace.h"
 
 #define ROCKSDB_MAJOR 6
-#define ROCKSDB_MINOR 19
+#define ROCKSDB_MINOR 20
 #define ROCKSDB_PATCH 0
 
 // Do not use these. We made the mistake of declaring macros starting with