Make go 1.21 the new minimum

[moleske]

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Where this PR should be backported?

• main - all changes should by default start here
• v8
• v7

Description of the Change

We must be able to understand the design of your change from this description.
Keep in mind that the maintainer reviewing this PR may not be familiar with or
have worked with the code here recently, so please walk us through the concepts.
make go 1.21 the new minimum

Why Is This PR Valuable?

What benefits will be realized by the code change? What users would want this change? What user need is this change addressing?
keep up to date on go versions

Applicable Issues

List any applicable GitHub Issues here

How Urgent Is The Change?

Is the change urgent? If so, explain why it is time-sensitive.

Other Relevant Parties

Who else is affected by the change?