feat: deprecate set_state for insert_state

[jbr]

I'm on the fence about this api churn, but this was a mistake that has been bothering me for a while, and trillium hasn't had intentional breaking changes since August 2021.

I'm going to hold off on releasing this until #611 is ready, just in case there are other easily-deprecatable changes, and then I'll stagger the releases by a week or so.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 31.57895% with 13 lines in your changes are missing coverage. Please review.

Project coverage is 48.83%. Comparing base (0bc2435) to head (3235c80).

Files	Patch %	Lines
websockets/src/websocket_connection.rs	0.00%	6 Missing :warning:
trillium/src/conn.rs	66.66%	2 Missing :warning:
channels/src/channel_central.rs	0.00%	1 Missing :warning:
proxy/src/upstream/connection_counting.rs	0.00%	1 Missing :warning:
testing/src/test_conn.rs	0.00%	1 Missing :warning:
trillium/src/state.rs	0.00%	1 Missing :warning:
websockets/tests/json.rs	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #637 +/- ## ========================================== - Coverage 48.85% 48.83% -0.03% ========================================== Files 183 183 Lines 7242 7245 +3 ========================================== Hits 3538 3538 - Misses 3704 3707 +3 ```

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