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/provider/windscribe/updater/api.go

@@ -3,7 +3,6 @@ package updater
 import (
 	"context"
 	"encoding/json"
-	"errors"
 	"fmt"
 	"net/http"
 	"net/netip"
@@ -11,8 +10,6 @@ import (
 	"time"
 )
 
-var ErrHTTPStatusCodeNotOK = errors.New("HTTP status code not OK")
-
 type apiData struct {
 	Data []regionData `json:"data"`
 }
@@ -55,7 +52,7 @@ func fetchAPI(ctx context.Context, client *http.Client) (
 	defer response.Body.Close()
 
 	if response.StatusCode != http.StatusOK {
-		return data, fmt.Errorf("%w: %d %s", ErrHTTPStatusCodeNotOK,
+		return data, fmt.Errorf("HTTP status code not OK: %d %s",
 			response.StatusCode, response.Status)
 	}