Consolidate legacy extensions

[jhollinger]

Merges most V2 "legacy extensions" into a single LegacyOptions. Two remain separate:

• LegacyDynamicOptions (more expensive and new, so not widely used)
• LegacyTransformer (it takes transformers as arguments)

class ApplicationBlueprint < Blueprinter::V2::Base
  # Legacy if, unless, default_if, name, extractor, and view-on-render options
  add Blueprinter::Extensions::LegacyOptions.new
end

class OtherBlueprint < ApplicationBlueprint
  # Legacy dynamic options
  add Blueprinter::Extensions::LegacyDynamicOptions.new

  # Legacy transformers
  add Blueprinter::Extensions::LegacyTransformer.new(MyTransformer)
end

Also adds a Performance page to the Building V2 Extensions section.

Drops test runs for Ruby 3.1 because it suddenly started throwing errors.

[jhollinger]

Merging bc this is a pretty clear win.