chore: do not use sentinel errors when unneeded

- main reason being it's a burden to always define sentinel errors at global scope, wrap them with `%w` instead of using a string directly
- only use sentinel errors when it has to be checked using `errors.Is`
- replace all usage of these sentinel errors in `fmt.Errorf` with direct strings that were in the sentinel error
- exclude the sentinel error definition requirement from .golangci.yml
- update unit tests to use ContainersError instead of ErrorIs so it stays as a "not a change detector test" without requiring a sentinel error

internal/server/middlewares/auth/lookup.go

@@ -21,7 +21,7 @@ func settingsToLookupMap(settings Settings) (routeToRoles map[string][]internalR
 		case AuthBasic:
 			checker = newBasicAuthMethod(role.Username, role.Password)
 		default:
-			return nil, fmt.Errorf("%w: %s", ErrMethodNotSupported, role.Auth)
+			return nil, fmt.Errorf("authentication method not supported: %s", role.Auth)
 		}
 
 		iRole := internalRole{