remove stalebot (#8466)
remove stalebot (#8466)

Bring Nomad in line with other HashiCorp projects and remove stalebot. We get
little value in cleaning up issues automatically this way, it adds extra work
for maintainers when we have issues waiting on the backlog that we intend to
do, and it presents an unkind experience to issue contributors who get their
issues closed by an impersonal bot.
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