Set go 1.21.0, remove toolchain 1.22 statement

[tesshuflower]

Describe what this PR does

Explicitly sets go 1.21.0 in go.mod and removes the "toolchain" statement that got automatically put there by a go mod tidy.

We don't want to force a go 1.22 toolchain at this point. It looks like with go 1.21 it's encouraged to set an X.Y.Z version as you need an explicit release version for the toolchain stuff to work. In this case go mod tidy added the x.y.z and also added a toolchain statement with the go version used (I assume?) because we picked up syncthing which itself had a go.mod with go 1.21.0.

So simply using go 1.21.0 in our go.mod means we can now run go mod tidy and it won't add a "toolchain" directive to the file on us.

This is better explained here: golang/go#62278 (comment)

Is there anything that requires special attention?

Related issues:

[openshift-ci]

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[sonarqubecloud]

Quality Gate passed

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0 New issues
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See analysis details on SonarCloud

[tesshuflower]

/cc @JohnStrunk

[JohnStrunk]

/lgtm
I found this post to be useful: Forward Compatibility and Toolchain Management in Go 1.21 - The Go Programming Language