Potentially increase go.mod version to 1.21 to provide slog support

[atc0005]

This could be useful for setting up package debug logging without requiring external dependencies.

Some useful references:

• https://betterstack.com/community/guides/logging/logging-in-go/
• https://reliasoftware.com/blog/structured-logging-with-golang-slog
• https://go.dev/blog/slog

[atc0005]

[!NOTE]

check-cert project is currently using Go 1.20, so we're looking at a Go version bump there also if we do this.