RestSharp update as Hotfix 2.12.1

[QilongTang]

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Purpose

RestSharp update as Hotfix 2.12.1. Cherry-picking #12165

Declarations

Check these if you believe they are true

• The codebase is in a better state after this PR
• Is documented according to the standards
• The level of testing this PR includes is appropriate
• User facing strings, if any, are extracted into *.resx files
• All tests pass using the self-service CI.
• Snapshot of UI changes, if any.
• Changes to the API follow Semantic Versioning and are documented in the API Changes document.
• This PR modifies some build requirements and the readme is updated

Release Notes

RestSharp update as Hotfix 2.12.1

Reviewers

@DynamoDS/dynamo

FYIs

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[QilongTang]

@mjkkirschner @pinzart90 @zeusongit @reddyashish I am merging to try building it and test