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/publicip/api/errors.go

@@ -2,9 +2,4 @@ package api
 
 import "errors"
 
-var (
-	ErrTokenNotValid   = errors.New("token is not valid")
-	ErrTooManyRequests = errors.New("too many requests sent for this month")
-	ErrBadHTTPStatus   = errors.New("bad HTTP status received")
-	ErrServiceLimited  = errors.New("service is limited")
-)
+var ErrTooManyRequests = errors.New("too many requests sent for this month")