refactor(routergroup): use isHTTPMethod function instead of regex for HTTP method validation

[1911860538]

This change replaces the regular expression validation of HTTP methods
with the dedicated isHTTPMethod function in the RouterGroup.Handle method,
ensures methods are in the predefined list rather than just uppercase letters.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.96%. Comparing base (3dc1cd6) to head (3e6e884).
⚠️ Report is 194 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4407      +/-   ##
==========================================
- Coverage   99.21%   98.96%   -0.25%     
==========================================
  Files          42       44       +2     
  Lines        3182     2907     -275     
==========================================
- Hits         3157     2877     -280     
- Misses         17       19       +2     
- Partials        8       11       +3     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.96% <100.00%> (?)
-tags go_json	98.89% <100.00%> (?)
-tags nomsgpack	98.95% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.24	98.96% <100.00%> (?)
go-1.25	98.96% <100.00%> (?)
macos-latest	98.96% <100.00%> (-0.25%)	⬇️
ubuntu-latest	98.96% <100.00%> (-0.25%)	⬇️

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