Remove "Upgrading from Old Versions" from Asset Pipeline Guide [skip ci]

[matildasmeds]

I would remove the chapter "Upgrading from Old Versions" from Asset Pipeline Guide altogether.

• Information on how to upgrade Rails 2.x/3.2 does not seem relevant for current Rails version.
• Also, all configuration changes mentioned are already included in Upgrading Ruby on Rails Guide,

[rails-bot]

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[gmcgibbon]

I think this makes sense. 👍 These upgrade docs seem primarily geared toward upgrading Rails 3 -> Rails 4.

[kamipo]

Thanks!