Update Release Version to 6.26 (#9059)

Summary:
Before cutting release branch 6.26, update version.h and release notes

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9059

Reviewed By: ajkr

Differential Revision: D31805126

fbshipit-source-id: ae85ccf06ec756fa21163161f53fd0b728e6e32e

HISTORY.md

@@ -1,5 +1,5 @@
 # Rocksdb Change Log
-## Unreleased
+## 6.26.0 (2021-10-20)
 ### Bug Fixes
 * Fixes a bug in directed IO mode when calling MultiGet() for blobs in the same blob file. The bug is caused by not sorting the blob read requests by file offsets.
 * Fix the incorrect disabling of SST rate limited deletion when the WAL and DB are in different directories. Only WAL rate limited deletion should be disabled if its in a different directory.

include/rocksdb/version.h

@@ -10,7 +10,7 @@
 #include "rocksdb/rocksdb_namespace.h"
 
 #define ROCKSDB_MAJOR 6
-#define ROCKSDB_MINOR 25
+#define ROCKSDB_MINOR 26
 #define ROCKSDB_PATCH 0
 
 // Do not use these. We made the mistake of declaring macros starting with