Chore: upgrade engines field to >=8

[mayurkale22]

We have already deprecated Node 6 support #511.

[codecov-io]

Codecov Report

Merging #533 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #533      +/-   ##
==========================================
+ Coverage   95.28%   95.28%   +<.01%     
==========================================
  Files         146      147       +1     
  Lines        9789     9877      +88     
  Branches      537      542       +5     
==========================================
+ Hits         9327     9411      +84     
- Misses        462      466       +4

Impacted Files	Coverage Δ
src/detect-resource.ts	90.9% <0%> (-9.1%)	⬇️
test/test-jaeger-format.ts	94.91% <0%> (-1.24%)	⬇️
test/test-detect-resource.ts	99.02% <0%> (-0.98%)	⬇️
src/http-stats.ts	100% <0%> (ø)	⬆️
src/binary-format.ts	100% <0%> (ø)	⬆️
src/grpc-stats/server-stats.ts	100% <0%> (ø)	⬆️
src/resource-labels.ts	100% <0%> (ø)	⬆️
src/grpc-stats/client-stats.ts	100% <0%> (ø)	⬆️
src/grpc-stats/stats-common.ts	100% <0%> (ø)	⬆️
src/grpc-stats/common-distributions.ts	100% <0%> (ø)

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