Fix forbidden API check: specify UTF-8 charset

The test was using String(byte[]) without specifying charset, which
is flagged by forbiddenapis as using the default charset.

Author: Chhida

modules/openapi-generator/src/test/java/org/openapitools/codegen/java/jersey3/JavaJersey3ErrorEntityTest.java

@@ -142,7 +142,7 @@ private String readTemplate(String templateName) throws Exception {
             java.io.InputStream is = getClass().getClassLoader()
                 .getResourceAsStream(JERSEY3_TEMPLATE_DIR + templateName);
             if (is != null) {
-                return new String(is.readAllBytes());
+                return new String(is.readAllBytes(), java.nio.charset.StandardCharsets.UTF_8);
             }
         } catch (Exception ignored) {}