docs: Create set_mute_undefined_finding sample

[hannahtsai2024]

Create set_mute_undefined_finding sample for use case: resetting an individual finding's mute state to UNDEFINED.

Description

Fixes #

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Checklist

• [x] I have followed Contributing Guidelines from CONTRIBUTING.MD
• [x] Tests pass: go test -v ./.. (see Testing)
• [x] Code formatted: gofmt (see Formatting)
• [x] Vetting pass: go vet (see Formatting)
• [x] These samples need a new API enabled in testing projects to pass (let us know which ones)
• [x] These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• [x] This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• [x] This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• [x] Please merge this PR for me once it is approved

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 1 region tag.

- [securitycenter/muteconfig/set_mute_undefined_finding.go:17](https://github.com/hannahtsai2024/golang-samples/blob/1b927c69578b52763bd5c32afb0f06dbfdee73a0/securitycenter/muteconfig/set_mute_undefined_finding.go#L17), tag `securitycenter_set_mute_undefined`

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[hannahtsai2024]

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