Prepare v3 release: module path bump and UTC time decoding

[shamaton]

• Bump module path to github.com/shamaton/msgpack/v3 and update all internal imports
• Change default time.Time decode behavior to UTC and align tests to UTC
• Update README for v3 install and upgrade guidance
• Raise Go version in go.mod and CI matrix (Codecov on 1.25)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.26%. Comparing base (8ee2364) to head (6e82c0c).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #57   +/-   ##
=======================================
  Coverage   99.26%   99.26%           
=======================================
  Files          72       72           
  Lines        5677     5677           
=======================================
  Hits         5635     5635           
  Misses         29       29           
  Partials       13       13           

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