Updating actions/setup-go to v6.3.0 from a mix of different versions.
This gets us faster CI runs, with improvements such as:
- built in Go module cache AND Go build cache (separate cache no longer
needed)
- using go.mod resulting in fewer cache invalidations
- faster Node 24 runtime
- using go.dev download URL, which is more reliable
<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit
* **Chores**
* Upgraded security protections across build and deployment workflows
for enhanced runner environment hardening.
* Strengthened CI/CD infrastructure security measures throughout
automated processes.
* No direct user-facing changes.
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settings.</sub>
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Done as part of oncall improvements.
`vars.GO_VERSION` can only be changed by admins and it's not public
(Fleet devs don't know the current value of the variable), this approach
uses the version specified in our `go.mod` file.
#19182
Releasing new fleetd-base flow
- The flow has been QA'd
- The flow puts the generated files into new directories (`stable` and
`archive`), so risk is low
