Ensure go version 1.21 in microk8s capi release

[HomayoonAlimohammadi]

Overview

This PR ensures go version 1.21 in Microk8s CAPI release job. The current line does not do what it's intended to because snap install exits with code 0 if the snap already exists so the snap refresh would never run.

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Mark below if this PR requires a job refresh (jjb) after merge:

• Needs jjb after merge

Please make sure to open PR's against the correct code:

• For integration tests, please make changes in jobs/integration
• For validation envs, jobs/validate
• For MicroK8s,jobs/microk8s
• For charm/bundle builds, jobs/build-charms

[addyess]

@HomayoonAlimohammadi will it ALWAYS be 1.21? Is there ever a chance it moves? Is there a source of truth for knowing the correct version?

[HomayoonAlimohammadi]

@addyess I've set the version according to Microk8s CAPI bootstrap provider and Microk8s CAPI control plane provider. That should be the source of truth for this. Theoretically speaking newer versions of Go should also work fine but that's not what I've experienced. Different versions of golangci-lint are not fully compatible with newer versions of Go. (e.g. AFAIK gonalgci-lint v1.57.2 has a memory leak when running with Go 1.23 and eventually crashes)
Probably the best solution for this is to have a go-version.txt in the projects acting as a source of truth (similar to the kubernetes stable.txt).
I'm not sure if go 1.21 will fix all the failing steps in the Microk8s CAPI release job, but I don't know of any way to test this. Is there any way to test if things work in a staging area?