Create set_mute_undefined_finding.go

Add code sample for resetting finding mute state to UNDEFINED.

Description

Fixes #<357870674> Create code sample that reset a Finding's mute state to UNDEFINED.

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