Deprecate tiers and use fully-qualified model variants.

[jcdyer]

Proposed changes

Deepgram no longer relies on explicit "tiers" to identify models. Instead, the tier name is included in the model param. This PR removes tiers, deprecates implicit-architecture usecase model variants, and adds variants for all current architectures.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update or tests (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING.md doc
• I have added tests and/or examples that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

[jcdyer]

Semver failure is expected. This will go out in a 0.5.0 release with a few other PRs.