chore(deps): update go version to 1.21.5

[loderunner]

Checklist

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• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have checked for similar issues and haven't found anything relevant.
• This is not a security issue (which should be reported here: https://circleci.com/security/)
• I have read Contribution Guidelines.

Internal Checklist

• I am requesting a review from my own team as well as the owning team
• I have a plan in place for the monitoring of the changes that I am making (this can include new monitors, logs to be aware of, etc...)

Changes

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• Update go version to 1.21.5
• Update dependencies
• Change macOS CI to use M1 resource class
• Update Xcode version
• Fix and simplify tests to remove odd usage of TESTING=true env var

Rationale

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Following a series of CVEs in Go, it is relevant to udpate Go version.