Switch version from function to string in otelecho

[jongwooo]

Change Version() func to const Version string in otelecho

Part of #8272

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.3%. Comparing base (937f0da) to head (12ad50e).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #8365     +/-   ##
=======================================
- Coverage   82.3%   82.3%   -0.1%     
=======================================
  Files        186     185      -1     
  Lines      13765   13763      -2     
=======================================
- Hits       11330   11328      -2     
  Misses      2030    2030             
  Partials     405     405             

Files with missing lines	Coverage Δ
...entation/github.com/labstack/echo/otelecho/echo.go	100.0% <100.0%> (ø)

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