chore: bump Go to 1.21

[knqyf263]

Description

Trivy has a policy supporting the last two major releases, just as Go does. This was mainly for reasons within our organization, but now that Go has introduced better forward compatibility in Go 1.21, it looks like we can remove this policy.
https://go.dev/blog/toolchain

This PR bumps Go to 1.21. After Go 1.21, we no longer need to care about the old stable version as Go automatically downloads the necessary toolchain. Therefore, this PR also updates some workflows.
https://go.dev/doc/toolchain

Related PRs

• feat(vex): Add support for CSAF format #5535

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[DmitriyLewen]

Perhaps we can also use go-mod-file for unit tests:

trivy/.github/workflows/test.yaml

Line 23 in 16b757d

go-version: oldstable

[knqyf263]

@DmitriyLewen Isn't this change?
https://github.com/aquasecurity/trivy/pull/5662/files#diff-245392b692a50c38ecab4381b118862db514035c10983f3bd4f4b7f1f4be4692R23

[DmitriyLewen]

hm... sorry for confusing...
I mixed up workflow files names 😄

[DmitriyLewen]

LGTM