Remove support for Environments

[zanieb]

note this merges into a 1.0.0 release candidate branch instead of master

Summary

Removes environments which have been deprecated for several versions.

Changes

Cherry-picked the following commits from #4612

• Remove deprecated FargateAgent
• Remove deprecated prefect.environments module
• Remove deprecated Flow.environment setting
• Remove deprecated Flow.environment serializers
• Add backwards compatibility for Flow.environment to flow diagnostics
• Update flow tests to reflect removal
• Update get_flow_image to use run_config instead of environment
• Oh, there are two get_flow_image functions -- deprecate the old looking one
• Remove environments from get_flow_run_command tests
• Remove environment docker healthcheck tests
• Remove environment serialization tests
• Remove environment handling from Client.register
• Remove environment handling from prefect build/register
• Update _get_run_config to drop support for environments
• Remove KubernetesAgent.generate_job_spec_from_environment
• Remove LocalEnvironment from DockerAgent tests
• Remove DockerAgent.network tests
• Remove remaining DockerAgent environment tests
• Drop some removed items from the API reference

Additional commits are quite granular and remove additional items from the documentation, fix tests, etc.

Importance

Removes an outdated configuration method. Replaced by RunConfig.