Fix a race condition when calling `go-licence-detector`. The original
code waited for a goroutine to write to `go-licence-detector`'s stdin
before starting the command, so the mage target was hanging.

This commit fixes it, adds logging (when verbose set) to the command
run as well as improves the error message.

Co-authored-by: Craig MacKenzie <[email protected]>

This commit fixes the call to `go-licence-detector` by removing the
`-depsOut ""` argument.

The NOTICE.txt file is also updated.

Our `Makefile` when running the `notice` rule appends some data to
`NOTICE.txt`, this commit makes the `notice` target on our
`magefile.go` to behave the same way.

The current implementation of mage notice is not working because it
was never finalised, the fact that it and `make notice` exist only
generates confusion.

This commit removes the `mage notice` and documents that `make notice`
should be used insted for the time being.

In the long run we want to use the implementation on
`elastic-agent-libs`, however it is not woking at the monent.

Closes elastic#1107