chore: add Stale to close old pull requests/issues

[jawnsy]

Add configuration for the Probot Stale bot, in order to close old
pull requests and issues.

[codecov]

Codecov Report

Merging #98 (44fddb8) into main (34fc62d) will increase coverage by 2.81%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main      #98      +/-   ##
==========================================
+ Coverage   69.44%   72.26%   +2.81%     
==========================================
  Files          84       89       +5     
  Lines        3404     3627     +223     
  Branches       55       55              
==========================================
+ Hits         2364     2621     +257     
+ Misses        851      788      -63     
- Partials      189      218      +29     

Flag	Coverage Δ
unittest-go-macos-latest	67.86% <ø> (-0.04%)	⬇️
unittest-go-ubuntu-latest	70.06% <ø> (?)
unittest-go-windows-latest	66.75% <ø> (?)
unittest-js	78.30% <ø> (ø)

Impacted Files	Coverage Δ
provisioner/terraform/serve.go	66.66% <0.00%> (ø)
provisioner/terraform/provision.go	57.62% <0.00%> (ø)
coderd/userpassword/userpassword.go	63.41% <0.00%> (ø)
provisioner/terraform/parse.go	75.00% <0.00%> (ø)
database/postgres/postgres.go	70.00% <0.00%> (ø)
peer/channel.go	84.75% <0.00%> (+1.82%)	⬆️
peer/conn.go	79.42% <0.00%> (+2.34%)	⬆️
coderd/coderdtest/coderdtest.go	100.00% <0.00%> (+22.64%)	⬆️
database/migrate.go	52.38% <0.00%> (+52.38%)	⬆️
database/pubsub.go	77.08% <0.00%> (+77.08%)	⬆️

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[kylecarbs]

Seems reasonable to me. Good hygiene!