Use correct go directive in go.mod file (#5720)

The version provided in the go directive must be a valid Go
version. go1.21 refers to a development version of Go, not a released
version. Prior to 1.21, 1.X was actually the first released version for
Go language version 1.X. Starting with 1.21, the first released version
is 1.X.0, and the go directive in go.mod should correspond to a valid
toolchain version (unless a toolchain directive is also present).

One way to reproduce the issue is to run:
`GOTOOLCHAIN="go1.20+auto" go version`

When using the `go 1.21` directive, the above command will fail as go
will try to download toolchain 1.21, which does not exist.

See golang/go#62278 for a detailed discussion.

Signed-off-by: Antonin Bas <[email protected]>

go.mod

@@ -1,6 +1,6 @@
 module antrea.io/antrea
 
-go 1.21
+go 1.21.0
 
 require (
 	antrea.io/libOpenflow v0.13.0