Clarify Version#rely_on_built_at? protects against other incongruent gem dates

[Daniel-N-Huss]

In discussing invalid dates appearing on some Gem::Specification in rubygems PR #6859, it was uncovered that this helper method has been inadvertently protecting RubyGems.org from displaying incorrect build dates in other contexts.

Originally used to handle importing gems into the service back in 2003, we discovered that some maintainers utilizing Nix ran into a case where default values for SOURCE_DATE_EPOCH were provided as the build date for the gem. rely_on_built_at? protected users on the website, or accessing gem details via the API, from seeing these incorrect dates.

@duckinator wisely suggested to add a comment here, surfacing the inadvertent benefit the method has been carrying. That comment also suggests more documentation around gem builds, and setting SOURCE_DATE_EPOCH which I'll leave for separate PR's (and link to, from here, if possible!)

[codecov]

Codecov Report

Merging #3993 (33e9119) into master (44a1ca9) will not change coverage.
Report is 8 commits behind head on master.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3993   +/-   ##
=======================================
  Coverage   98.91%   98.91%           
=======================================
  Files         228      228           
  Lines        5550     5550           
=======================================
  Hits         5490     5490           
  Misses         60       60           

Files Changed	Coverage Δ
app/models/version.rb	98.12% <ø> (ø)