[go.mod] Change go directive to 1.21

On b3432fb, the minimum supported
Go version was updated to `1.21.0`. This changes the version to `1.21`
so that we are using a language version. If not using toolchain, this
seems to be the preferred convention in the ecosystem and golang/go.

Signed-off-by: Pablo Baeyens <pablo.baeyens@datadoghq.com>

go.mod

@@ -1,6 +1,6 @@
 module github.com/cilium/ebpf
 
-go 1.21.0
+go 1.21
 
 require (
 	github.com/go-quicktest/qt v1.101.0