docs: Bump the supported version of Go to 1.21

[eskultety]

Maintainers will complete the following section

• [x] Commit messages are descriptive enough
• [x] Code coverage from testing does not decrease and new code is covered
• [x] Docs updated (if applicable)
• [x] Docs links in the code are still valid (if docs were updated)

Note: if the contribution is external (not from an organization member), the CI pipeline will not run automatically. After verifying that the CI is safe to run:

• approve GitHub Actions workflows by clicking a button
• approve the Red Hat Trusted App Pipeline container build by commenting /ok-to-test (as is the standard for Pipelines as Code)