* Add Ruby 3.1 to CI

Update Rubocop for recent Rubies
Disable Rubocop run for Rubies before Ruby 2.4
Quote '3.0' in the CI configuration to ensure it loads a 3.0.x Ruby
Set RUBYOPT="--disable_error_highlight" so Ruby 3.1 error matchers pass

* Add CHANGELOG.md entry

* Re-add deleted line from CHANGELOG.md

* Set minimum supported Ruby version to 2.4.

Remove a number of code bits designed to support Rubies below version 2.4

* Bump version.  Remove unneeded require from Gemfile.  Add require to spec/support file

`Mash.load` uses the Ruby standard library to load Yaml-serialized files
into a Mash. The original implementation used `YAML.load` for this
purpose. However, that method is inherently unsafe so we switched to
using `YAML.safe_load`.

Safely loading Yaml files has many different domain-specific
configuration flags that we did not, by default, expose. This change
introduces the ability to configure the safe loading of Yaml files so
that all types of Yaml can be loaded when necessary using the flags from
the standard library.

This implementation preserves the backwards-compatibility with the prior
implementation so that it should not require updates from users of the
current `Mash.load` behavior. For those who this change affects, we
included upgrading documentation to ease the transition.

This is a big step forward in our Rubocop setup. I addressed all of the todos
from the current version of Rubocop that made sense to. The only things that
remain are metrics and one cop that relies on the line length metric to work.

I made some judgment calls on disabling a few cops:

1. The `Layout/IndentHeredoc` cop wants you to either use the squiggly heredoc
   from Ruby 2.3 or introduce a library. Since we are a low-level library that
   is used as a transitive dependency, we cannot introduce another library as a
   dependence, so that option is out. Also, we support Rubies back to 2.0
   currently, so using the squiggly heredoc isn't an option. Once we remove
   support for Rubies older than 2.3, we can switch to the squiggly heredoc cop.
2. The `Naming/FileName` cop was reporting false positives for a few files in
   the repository, so I disabled it on those files.
3. The `Style/DoubleNegation` cop reports lints on a few cases where we use
   double negation. Given the very generic nature of Hashie, the double-negation
   is the easiest, clearest way to express that we want an item to be a Boolean.
   I disabled the cop because we exist in the gray area where this makes sense.