Update default BBTO::format_version from 2 to 4 (#6582)

Summary:
Version 4 has been around long enough, for compatibility and
extensive validation, that it should be default.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6582

Test Plan:
CI (w.r.t. changing the default; format_version=4 is well
tested and massively in production at Facebook)

Reviewed By: siying

Differential Revision: D20625233

Pulled By: pdillinger

fbshipit-source-id: 2f83ed874cffa4a39bc7a66cdf3833b978fbb948

HISTORY.md

@@ -2,6 +2,7 @@
 ## Unreleased
 ### Public API Change
 * Fix spelling so that API now has correctly spelled transaction state name `COMMITTED`, while the old misspelled `COMMITED` is still available as an alias.
+* Updated default format_version in BlockBasedTableOptions from 2 to 4. SST files generated with the new default can be read by RocksDB versions 5.16 and newer, and use more efficient encoding of keys in index blocks.
 
 ### Bug Fixes
 * Fix a bug where range tombstone blocks in ingested files were cached incorrectly during ingestion. If range tombstones were read from those incorrectly cached blocks, the keys they covered would be exposed.

include/rocksdb/table.h

@@ -272,7 +272,7 @@ struct BlockBasedTableOptions {
   // 5 -- Can be read by RocksDB's versions since 6.6.0. Full and partitioned
   // filters use a generally faster and more accurate Bloom filter
   // implementation, with a different schema.
-  uint32_t format_version = 2;
+  uint32_t format_version = 4;
 
   // Store index blocks on disk in compressed format. Changing this option to
   // false  will avoid the overhead of decompression if index blocks are evicted