Use toolchain directive to declare preferred Go version

Use the go directive to define the minimum supported Go version, and the
toolchain directive to select the preferred Go version. This makes it
nicer for downstream consumers, so they can still depend on sonobuoy and
build their software with older Go patch releases, and can upgrade their
toolchains at their own pace.

Signed-off-by: Tom Wieczorek <twieczorek@mirantis.com>

Author: twz123

go.mod

@@ -1,6 +1,8 @@
 module github.com/vmware-tanzu/sonobuoy
 
-go 1.26.4
+go 1.26.0
+
+toolchain go1.26.4
 
 require (
 	github.com/briandowns/spinner v1.19.0