Deprecate PrefectAgent and prefect agent command group

[desertaxle]

Adds deprecation warnings to the PrefectAgent class and prefect agent command group. Users will receive a recommendation to use workers and a link to the workers upgrade guide.

Example

.venv ~/dev/PrefectHQ/prefect git:(deprecate/agent)
prefect agent start -q test
WARNING: The 'agent' command group has been deprecated. It will not be available after Sep 2024. Use `prefect worker start` instead. Refer to the upgrade guide for more information: https://docs.prefect.io/latest/guides/upgrade-guide-agents-to-workers/.
Starting v2.16.3+7.g4fe3951e0 agent with ephemeral API...

  ___ ___ ___ ___ ___ ___ _____     _   ___ ___ _  _ _____
 | _ \ _ \ __| __| __/ __|_   _|   /_\ / __| __| \| |_   _|
 |  _/   / _|| _|| _| (__  | |    / _ \ (_ | _|| .` | | |
 |_| |_|_\___|_| |___\___| |_|   /_/ \_\___|___|_|\_| |_|


Agent started! Looking for work from queue(s): test...

Checklist

• This pull request references any related issue by including "closes <link to issue>"
  • If no issue exists and your change is not a small fix, please create an issue first.
• This pull request includes tests or only affects documentation.
• This pull request includes a label categorizing the change e.g. maintenance, fix, feature, enhancement, docs.

For documentation changes:

• This pull request includes redirect settings in netlify.toml for files that are removed or renamed.

For new functions or classes in the Python SDK:

• This pull request includes helpful docstrings.
• If a new Python file was added, this pull request contains a stub page in the Python SDK docs and an entry in mkdocs.yml navigation.

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[serinamarie]

LGTM!

[serinamarie]

👏