Fix go directive version in go.mod

[mauri870]

According to https://go.dev/issues/62278 a concrete go version has to include the patch number or an additional toolchain directive. The project declares the version as 1.21 which is invalid and will result in an error:

GOTOOLCHAIN="go1.20+auto" go1.21.5 mod tidy
go: downloading go1.21 (linux/amd64)
go: download go1.21 for linux/amd64: toolchain not available

Changed to 1.21.0 as this is the first actual release of go 1.21.

[cla-checker-service[bot]]

💚 CLA has been signed

[Anaethelion]

Thank you! I'll update the relevant go.mod files to make sure the CI is up to date.