chore: drop node 9 from CircleCI

[faustbrian]

Proposed changes

Dropping node 9 as node 11 has been released so node 10 will be our minimum requirement. I will send another PR to require node 10 as minimum for core packages.

Types of changes

• Other

Checklist

• I have read the CONTRIBUTING documentation
• Lint and unit tests pass locally with my changes

[codecov-io]

Codecov Report

Merging #1245 into develop will decrease coverage by 0.08%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #1245      +/-   ##
===========================================
- Coverage    78.72%   78.63%   -0.09%     
===========================================
  Files          415      415              
  Lines         6966     6966              
  Branches       929      929              
===========================================
- Hits          5484     5478       -6     
- Misses        1310     1316       +6     
  Partials       172      172

Impacted Files	Coverage Δ
...p2p/lib/server/versions/internal/handlers/utils.js	80% <0%> (-20%)	⬇️
packages/core-forger/lib/manager.js	71.73% <0%> (-2.18%)	⬇️
packages/core-blockchain/lib/state-machine.js	45% <0%> (-0.56%)	⬇️

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