Fix deprecation warning regarding implicit injections

[mayatron]

The following deprecation warning appears on application launch, starting with Ember 3.26.0:

DEPRECATION: A value was injected implicitly on the 'emberSpinnerPrefixConfig' property of an instance of <monolith@component:ember-spinner::ember195>. Implicit injection is now deprecated, please add an explicit injection for this value. If the injected value is a service, consider using the @service decorator. [deprecation id: implicit-injections] See https://deprecations.emberjs.com/v3.x#toc_implicit-injections for more details.

This appears to be caused by:

https://github.com/kiwiupover/ember-cli-spinjs/blob/667b1f128a1993687d31c0695c4dd5c43f2d6c89/app/initializers/ember-spinner.js#L7-L15