Update rubocop for ruby 2.7+

[p-datadog]

What does this PR do?
Updates rubocop and related dependencies to newer versions.

Removes rubocop from ruby 2.6 configuration since the current versions of rubocop do not support ruby 2.6.
Removes rubocop from jruby 9.3 configuration because jruby 9.3 identifies as ruby 2.6 and hence rubocop does not support it anymore.
Removes standard from rubies that do not have rubocop, because then it would bring an old rubocop that wouldn't behave the same as the one in supported versions.
Updates rubocop to 1.71. Current version of rubocop is 1.73 but standard does not support 1.73 yet.
standard version is not explicitly specified but it gets updated to 1.45 from somewhere in the 1.20 range.
Several files edited for changes demanded by recent rubocops.
rubocop-rspec updated to most recent 2.x release, 3.0 has many new demands producing a huge diff likely to conflict with other PRs.

Motivation:

Noticed old version when testing rubocop/rubocop#13933

Change log entry
None

Additional Notes:

Since rubocop, standard and related libraries need to have synchronized versions, this PR does not update rubocop to its latest version (currently 1.73, PR updates to 1.71) and e.g. rubocop-performance needs to be declared as 1.20 not 1.23.

rubocop-rspec 3.x wants to make MANY changes to our test suite, they will probably conflict with in-progress PRs.

How to test the change?
Existing CI

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.61905% with 1 line in your changes missing coverage. Please review.

Project coverage is 97.68%. Comparing base (9ea35ba) to head (99aa915).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...cing/contrib/mongodb/regression_issue_1235_spec.rb	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4448      +/-   ##
==========================================
- Coverage   97.69%   97.68%   -0.02%     
==========================================
  Files        1381     1381              
  Lines       83955    83954       -1     
  Branches     4253     4249       -4     
==========================================
- Hits        82024    82011      -13     
- Misses       1931     1943      +12     

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[datadog-datadog-prod-us1]

Datadog Report

Branch report: update-rubocop
Commit report: 99aa915
Test service: dd-trace-rb

✅ 0 Failed, 20631 Passed, 1371 Skipped, 3m 26.05s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-03-12 18:40:20

Comparing candidate commit 99aa915 in PR branch update-rubocop with baseline commit 9ea35ba in branch master.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 31 metrics, 2 unstable metrics.