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/vyprvpn/updater/filename.go

@@ -1,19 +1,16 @@
 package updater
 
 import (
-	"errors"
 	"fmt"
 	"strings"
 )
 
-var errNotOvpnExt = errors.New("filename does not have the openvpn file extension")
-
 func parseFilename(fileName string) (
 	region string, err error,
 ) {
 	const suffix = ".ovpn"
 	if !strings.HasSuffix(fileName, suffix) {
-		return "", fmt.Errorf("%w: %s", errNotOvpnExt, fileName)
+		return "", fmt.Errorf("filename does not have the openvpn file extension: %s", fileName)
 	}
 
 	region = strings.TrimSuffix(fileName, suffix)