Deprecate and provide replacements for inconsistent methods

[imjoehaines]

Goal

For consistency with Bugsnag notifiers for other languages, a number of methods have been deprecated in this PR. The old options will be removed in the next major version

• The notify_release_stages configuration option has been deprecated in favour of enabled_release_stages
• The auto_capture_sessions and track_sessions configuration options have been deprecated in favour of auto_track_sessions
• The Report class has been deprecated in favour of the Event class
• The Report#meta_data attribute has been deprecated in favour of Report#metadata
• The Breadcrumb#meta_data attribute has been deprecated in favour of Breadcrumb#metadata
• The Breadcrumb#name attribute has been deprecated in favour of Breadcrumb#message
• The breadcrumb type constants in the Bugsnag::Breadcrumbs module has been deprecated in favour of the constants available in the Bugsnag::BreadcrumbType module
  For example, Bugsnag::Breadcrumbs::ERROR_BREADCRUMB_TYPE is now available as Bugsnag::BreadcrumbType::ERROR

Design

I've avoided using the alias keyword as YARD will merge the alias into the original attribute, e.g the "also" here:

This makes it impossible to mark the original name as deprecated without also marking the new name as deprecated¹:

Therefore I've implemented the aliases manually to give them their own section in the generated API docs:

The new methods use the @!attribute directive so YARD generates the docs as if they were attributes and not methods

¹ I believe you can do it as it's a behaviour of the templates YARD uses, but usually aliases being merged is correct so it doesn't seem worth writing new templates just for this PR