chore: make go-lint happy

[panpan0000]

Closes #404

📑 Description

Fix part of the lint errors.
But I didn't fix them all

(1)Some health relevant code and unused functions I believe will be refine later by their authors?
(2) For code cmd/generate/generate.go:43, I don't understand what it means by override the default backend if a flag is provided.

So there are some issues left as below, later when they are all fixed, we can enable lint test in CI as PR Gate:

 make lint
INFO [lintersdb] Active 7 linters: [errcheck gosimple govet ineffassign staticcheck typecheck unused]

cmd/generate/generate.go:43:4: ineffectual assignment to backendType (ineffassign)
			backendType = backend
			^
pkg/server/server.go:41:2: field `maxConcurrency` is unused (unused)
	maxConcurrency int
	^
pkg/server/server.go:53:5: var `health` is unused (unused)
var health = Health{
    ^
pkg/server/server.go:104:18: func `(*Config).healthzHandler` is unused (unused)
func (s *Config) healthzHandler(w http.ResponseWriter, r *http.Request) {
                 ^
pkg/server/server.go:113:6: func `getBoolParam` is unused (unused)
func getBoolParam(param string) bool {
     ^
cmd/serve/serve.go:100:20: Error return value of `logger.Sync` is not checked (errcheck)
		defer logger.Sync()
		                 ^
INFO File cache stats: 3 entries of total size 8.9KiB

✅ Checks

• My pull request adheres to the code style of this project
• My code requires changes to the documentation
• I have updated the documentation as required
• All the tests have passed

ℹ Additional Information

[AlexsJones]

Great contribution thank you