stream: remove legacy prependListener fallback

[karan-lrn]

The prependListener method has been available on EventEmitter since Node.js v6.0.0 (April 2016). The fallback code that manipulated the internal _events object is no longer necessary.

This also removes tests that explicitly tested the fallback behavior.

[nodejs-github-bot]

Review requested:

• @nodejs/streams

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.69%. Comparing base (1ea93c7) to head (73f182e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #62435      +/-   ##
==========================================
+ Coverage   89.59%   89.69%   +0.09%     
==========================================
  Files         676      676              
  Lines      206581   206724     +143     
  Branches    39512    39593      +81     
==========================================
+ Hits       185094   185412     +318     
+ Misses      13583    13460     -123     
+ Partials     7904     7852      -52     

Files with missing lines	Coverage Δ
lib/internal/streams/legacy.js	97.32% <100.00%> (+2.87%)	⬆️

... and 47 files with indirect coverage changes

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[karan-lrn]

cc @nodejs/streams @mcollina @ronag

This PR removes the legacy prependListener fallback in lib/internal/streams/legacy.js. The fallback code manipulated the internal _events object to support Node.js versions before v6.0.0 (April 2016) where prependListener didn't exist.

Since Node.js v6 has been EOL for over 8 years, this cleanup removes ~15 lines of unnecessary code and the associated tests that explicitly tested the fallback behavior.

[mcollina]

It would be better to deprecate this first