issue: #42937

/kind branch-feature

## Summary
- Update Knowhere dependency from `v2.6.17` to `v2.6.18`.
- Use the Knowhere `v2.6.18` tag from the 2.6 branch:
https://github.com/zilliztech/knowhere/tree/v2.6.18

## Changes included in v2.6.18 (since v2.6.17)
- fix: handle int8 cosine normalization for GPU CAGRA
(zilliztech/knowhere#1724)

> Note: this is a 2.6-branch-specific dependency bump; knowhere 2.6 is
not tracked on milvus master (master follows knowhere v3.x), so there is
no corresponding master PR.

## Test plan
- [x] `git diff --check upstream/2.6..HEAD`
- [x] `git ls-remote --tags https://github.com/zilliztech/knowhere.git
refs/tags/v2.6.18`

Signed-off-by: marcelo-cjl <marcelo.chen@zilliz.com>