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]

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 ⚠️
trillium/src/conn.rs	66.66%	2 Missing ⚠️
channels/src/channel_central.rs	0.00%	1 Missing ⚠️
proxy/src/upstream/connection_counting.rs	0.00%	1 Missing ⚠️
testing/src/test_conn.rs	0.00%	1 Missing ⚠️
trillium/src/state.rs	0.00%	1 Missing ⚠️
websockets/tests/json.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            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     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.