bump version to v1.8.1

[xiangjianmeng]

• Targeted PR against correct branch (see CONTRIBUTING.md)

• [ ] Linked to github-issue with discussion and accepted design OR link to spec that describes this work.

• [ ] Wrote tests

• [ ] Updated relevant documentation (docs/)

• [ ] Added a relevant changelog entry to the Unreleased section in CHANGELOG.md

• [ ] Reviewed Files changed in the github PR explorer

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For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge PR #XYZ: [title]" (coding standards)