Revert to original upstream tklv/rocksdb fork (tikv/6.29)

For the underlying rocksdb repo (not the rust-rocksdb Rust bindings),
there is no longer any difference between our fork
(6.29.tikv-ingest_external_files) and TiKV's branch we forked from:

```
❯ git diff 20225a41c..arthurprs/6.29.tikv-ingest_external_files
<EOF>
```

Change back to the original branch, and pin to the last merged tikv/6.29 SHA (20225a41c).

Author: Dave Rigby

.gitmodules

@@ -1,7 +1,7 @@
 [submodule "rocksdb"]
 	path = librocksdb_sys/rocksdb
-	url = https://github.com/arthurprs/rocksdb.git
-	branch = 6.29.tikv-ingest_external_files
+	url = https://github.com/tikv/rocksdb.git
+	branch = 6.29.tikv
 
 [submodule "titan"]
 	path = librocksdb_sys/libtitan_sys/titan