chore: bump client/v2 to go.mod 1.21

[julienrbrt]

Description

Bumps client/v2 to go 1.21 as it depends on the SDK.
Done as well for v0.50: #17453

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Author Checklist

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I have...

• included the correct type prefix in the PR title
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• included the necessary unit and integration tests
• added a changelog entry to CHANGELOG.md
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