test: Increase PyMilvus version to 2.6.11rc18 for 2.6 branch

[pymilvus-bot]

Automated daily bump from pymilvus 2.6 branch. Updates tests/python_client/requirements.txt.

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[mergify]

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[INFO] PR Label Summary by Default
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• Target branch '2.6' requires a PR merged to master first
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• PR: test: Increase PyMilvus version to 2.6.11rc18 for 2.6 branch #48541
• Title: test: Increase PyMilvus version to 2.6.11rc18 for 2.6 branch
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[sre-ci-robot]

[ci-v2-notice]
Notice: New ci-v2 system is enabled for this PR.

To rerun ci-v2 checks, comment with:

• /ci-rerun-code-check // for ci-v2/code-check
• /ci-rerun-build // for ci-v2/build
• /ci-rerun-build-all // for ci-v2/build-all (multi-arch builds)
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