switch core to standard from rubocop

[p-datadog]

What does this PR do?

Switches core to use standard instead of rubocop

Motivation:
We decided to standardize on standard

Change log entry
None

Additional Notes:

How to test the change?

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.71038% with 17 lines in your changes missing coverage. Please review.

Project coverage is 97.73%. Comparing base (7de1732) to head (fc10668).

Files with missing lines	Patch %	Lines
lib/datadog/core/remote/client.rb	52.38%	10 Missing ⚠️
lib/datadog/core/utils/duration.rb	86.66%	2 Missing ⚠️
lib/datadog/core/utils/time.rb	66.66%	2 Missing ⚠️
lib/datadog/core/environment/container.rb	95.00%	1 Missing ⚠️
lib/datadog/core/environment/ext.rb	50.00%	1 Missing ⚠️
lib/datadog/core/metrics/logging.rb	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4677      +/-   ##
==========================================
- Coverage   97.73%   97.73%   -0.01%     
==========================================
  Files        1455     1455              
  Lines       87359    87360       +1     
  Branches     4490     4492       +2     
==========================================
- Hits        85382    85380       -2     
- Misses       1977     1980       +3     

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[datadog-datadog-prod-us1]

Datadog Report

Branch report: standard-core
Commit report: fc10668
Test service: dd-trace-rb

✅ 0 Failed, 21296 Passed, 1373 Skipped, 4m 6.55s Total Time

[Strech]

Awesome 👍🏼