[chronik] Bump the RocksDb crate version to 0.21.0

Summary: This fixes the build with recent Clang versions that used to fail for a missing stdint header inclusion.

Test Plan:

mkdir buildChronik && cd buildChronik
cmake -GNinja .. -DBUILD_BITCOIN_CHRONIK=ON
ninja all check-crates
./test/functional/test_runner.py chronik_*

Reviewers: #bitcoin_abc, bytesofman, tobias_ruck

Reviewed By: #bitcoin_abc, bytesofman, tobias_ruck

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14327