feat: Make serde dep user side obsolete

[jawoznia]

Removed serde and cw_multi_test dependency from examples.
Fixed features in examples as some already non existent features were used in cfg checks. Also mt feature now enables sylvia/mt instead of cw_multi_test.

[codecov]

Codecov Report

Attention: Patch coverage is 30.00000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 70.08%. Comparing base (e8ede74) to head (9fdf78a).
Report is 1 commits behind head on main.

Files	Patch %	Lines
...a-derive/src/contract/communication/wrapper_msg.rs	0.00%	5 Missing ⚠️
...ia-derive/src/contract/communication/struct_msg.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #422      +/-   ##
==========================================
- Coverage   70.12%   70.08%   -0.05%     
==========================================
  Files          53       53              
  Lines        3080     3085       +5     
==========================================
+ Hits         2160     2162       +2     
- Misses        920      923       +3     

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[kulikthebird]

Nice! Two comments to fix and we're good to go