chore: improve linting, testing, and GitHub Actions setup (#3583)

- Update golangci-lint version from `v1.48.0` to `v1.52.2`
- Remove Gitter notifications from GitHub Actions workflow
- Add gosec linter settings and include specific rules
- Exclude revive linter for test files
- Remove Gitter badge from README.md
- Delete codecov.yml file
- Change function parameter name in fs.go
- Remove unused parameter in defaultHandleRecovery function

Signed-off-by: appleboy <appleboy.tw@gmail.com>

fs.go

@@ -39,7 +39,7 @@ func (fs onlyFilesFS) Open(name string) (http.File, error) {
 }
 
 // Readdir overrides the http.File default implementation.
-func (f neuteredReaddirFile) Readdir(count int) ([]os.FileInfo, error) {
+func (f neuteredReaddirFile) Readdir(_ int) ([]os.FileInfo, error) {
 	// this disables directory listing
 	return nil, nil
 }