Fix MultiJson v1.21.0+ deprecation warnings with fallback for older versions

[larouxn]

Description

Due to v1.21.0 of the MultiJson gem deprecating a lot of things, deprecation warnings have begun appearing from this gem. Thus, this PR is aiming to address said deprecation warnings when peeps are using v1.21.0 of the MultiJson gem and simply fallback to existing behaviour when they're using older versions of it.

How?

Migrates to the new MultiJSON constant with generate/parse when available, falling back to the legacy MultiJson with dump/load for older versions.

Testing

Before

The MultiJson constant is deprecated and will be removed in v2.0. Use MultiJSON instead.
MultiJSON.load is deprecated and will be removed in v2.0. Use MultiJSON.parse instead.
MultiJSON.dump is deprecated and will be removed in v2.0. Use MultiJSON.generate instead.

After

No deprecation warnings.