End of an era: support ending for Rack 1.x

This is intended to be the final release of rack-contrib with support for
the Rack 1.x series. Deprecations have been added for the
non-backwards-compatible changes, so you can get a sense of what will need
to be fixed.

Regular changes in this release:

• Rack::Locale: use a default locale (specified by I18n.default_locale = :<something>) if no available locale matches the Accept header.
  (Hiroki Yoshioka)

• Our test suite is now minitest 6 compliant, since all deprecation warnings
  have been cleaned up. (Skye Shaw)

Deprecation warnings added:

• Rack::Sendfile is gone, because there's a better one in Rack itself. If
  your code loads rack/contrib/sendfile, you'll get a warning.

• Rack::NestedParams behaviour has changed if multiple instances of the same
  variable are found in a query string (foo=1&foo=2 currently evaluates to
  foo=1; it will evaluate to foo=2 in rack-contrib 2.x). This is
  necessary because we're using Rack's built-in query string parsing rather
  than our own rather ugly implementation. If you hit the changed
  behaviour, you'll get a warning. (Skye Shaw)

• Rack::AcceptFormat -- whilst this has been deprecated for a while, it's
  worth reiterating that if you're still using this thing, you should really
  stop. It's sooooo bad.