Fix Java 8 build: use URLEncoder String charset

The follow-up that pre-encodes the OIDC form params changed
urlEncode() to URLEncoder.encode(value, StandardCharsets.UTF_8).
That Charset overload is @SInCE 10, so the source-of-truth JDK 8
build fails to compile: "incompatible types: Charset cannot be
converted to String" at OidcDeviceAuth.java:896.

Revert urlEncode() to the Java 8 String-charset form,
URLEncoder.encode(value, "UTF-8") with the
UnsupportedEncodingException catch, and drop the now-unused
StandardCharsets import. The constructor-time pre-encoding of
clientId/scope/audience/device_code is unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: glasstiger

core/src/main/java/io/questdb/client/cutlass/auth/OidcDeviceAuth.java

@@ -45,8 +45,8 @@
 import io.questdb.client.std.str.DirectUtf8Sequence;
 import io.questdb.client.std.str.StringSink;
 
+import java.io.UnsupportedEncodingException;
 import java.net.URLEncoder;
-import java.nio.charset.StandardCharsets;
 import java.util.concurrent.locks.ReentrantLock;
 
 /**
@@ -892,8 +892,13 @@ private static boolean settingsChannelIsPlaintext(Endpoint server) {
     }
 
     private static String urlEncode(String value) {
-        // the Charset overload is Java 10; the client targets Java 8, so use the String-charset form
-        return URLEncoder.encode(value, StandardCharsets.UTF_8);
+        try {
+            // the Charset overload is Java 10; the client targets Java 8, so use the String-charset form
+            return URLEncoder.encode(value, "UTF-8");
+        } catch (UnsupportedEncodingException e) {
+            // UTF-8 is guaranteed present on every JVM, so this is unreachable; rethrow defensively
+            throw new OidcAuthException(e).put("UTF-8 encoding is not supported");
+        }
     }
 
     private static void validateEndpointOrigins(Endpoint tokenEndpoint, Endpoint deviceAuthorizationEndpoint, Endpoint issuer) {