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/ipvanish/updater/filename.go

@@ -1,7 +1,6 @@
 package updater
 
 import (
-	"errors"
 	"fmt"
 	"strings"
 
@@ -9,8 +8,6 @@ import (
 	"golang.org/x/text/cases"
 )
 
-var errCountryCodeUnknown = errors.New("country code is unknown")
-
 func parseFilename(fileName, hostname string, titleCaser cases.Caser) (
 	country, city string, err error,
 ) {
@@ -28,7 +25,7 @@ func parseFilename(fileName, hostname string, titleCaser cases.Caser) (
 	countryCode := strings.ToLower(parts[0])
 	country, ok := countryCodes[countryCode]
 	if !ok {
-		return "", "", fmt.Errorf("%w: %s", errCountryCodeUnknown, countryCode)
+		return "", "", fmt.Errorf("country code is unknown: %s", countryCode)
 	}
 	country = titleCaser.String(country)