knowhere-v1.3.21

What's Changed

• Add async thread pool for generating diskann cache and catch unexpected return. by @cqy123456 in #226
• Change async thread pool size to 1 by @cqy123456 in #234
• Add some log for diskann prepare by @cqy123456 in #235
• Fix diskann async cache by @liliu-z in #250

Full Changelog: v1.3.20...v1.3.21