Add patch version number in go.mod (X.Y => X.Y.Z) (#68)

> Before Go 1.21, the initial release of a Go toolchain was
> version 1.N, not 1.N.0, so for N < 21, the ordering is
> adjusted to place 1.N after the release candidates.
>
> https://go.dev/doc/toolchain#version

Go changed its versioning scheme in 1.21 from `<major>.<minor>`
to `<major>.<minor>.<patch>`.  CodeQL analysis flags this
stating the lack of a patch number "may cause some `go` commands
to fail."

go.mod

@@ -1,6 +1,6 @@
 module github.com/adobe/aquarium-fish
 
-go 1.21
+go 1.21.0
 
 require (
  github.com/alessio/shellescape v1.4.1