fix: return index.html for /swagger and /swagger/ instead of 404

[illionillion]

Describe the PR

Fixes the issue where accessing /swagger and /swagger/ returned 404. Now, both paths return the content of index.html directly, matching typical static web server behavior and improving UX.

Closes #323

Additional context:

• Both /swagger and /swagger/ now return the Swagger UI directly instead of 404.
• Added tests for both paths to ensure future compatibility, in case gin's routing behavior changes.
• For local testing, please update example/basic/main.go as follows to ensure Swagger UI loads the API definition correctly:

- url := ginSwagger.URL("http://petstore.swagger.io:8080/swagger/doc.json") // The url pointing to API definition
+ url := ginSwagger.URL("/swagger/doc.json") // The url pointing to API definition

• Please check the behavior by accessing both /swagger and /swagger/ after applying this PR.

[illionillion]

@easonlin404 @pei0804 @sdghchj

Hi, just a small ping in case this PR was missed 🙂
It’s a minor change, so no rush at all. Thanks!

[sdghchj]

test with go 1.21.x failed.

[illionillion]

@sdghchj
I noticed that the CI was failing due to an outdated version of actions/setup-go, which does not support Go 1.21+.
I have updated the workflow to use actions/setup-go@v4 to resolve this issue.

Could you please approve and re-run the GitHub Actions for this PR?
Thank you!