fix: replace deprecated codec.ProtoMarshaler interface with proto.Message

[MozirDmitriy]

Description

• In testing/utils.go, replaced the parameter type in the UnmarshalMsgResponses function from codec.ProtoMarshaler to proto.Message and added the necessary import
• In e2e/testsuite/codec.go, also replaced the parameter type in the identically named function

Replacing the deprecated interface helps avoid potential issues in the future when codec.ProtoMarshaler might be completely removed from the codebase. Using the recommended proto.Message interface aligns with SDK development best practices.
The interface replacement maintains the same functionality as the implementation of the functions doesn't use any specific methods of the codec.ProtoMarshaler interface that aren't available in proto.Message.

---

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• Added relevant godoc comments.
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[gjermundgaraba]

Sorry for the long wait here! Could you please also add a changelog entry for this under ### Testing API in CHANGELOG.md?

Otherwise, I think this looks good and useful 🙏 Thank you!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.17%. Comparing base (2032604) to head (f8b1331).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8366   +/-   ##
=======================================
  Coverage   67.17%   67.17%           
=======================================
  Files         280      280           
  Lines       19429    19429           
=======================================
  Hits        13052    13052           
  Misses       5794     5794           
  Partials      583      583           

Flag	Coverage Δ
08-wasm	44.28% <ø> (ø)
ibc-go	68.87% <ø> (ø)

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