chore(bump): bump java version to 21

- kafka client: 3.3.1 -> 3.8.0
- aspectj: 1.9.9.1 -> 1.9.21

Maven Plugins:
- clean: 3.2.0 -> 3.4.0
- compile: 3.10.1 -> 3.13.0
- resources: 3.2.0 -> 3.3.1
- surefire: 3.1.2 -> 3.5.1
- openapi generator : 6.6.0 -> 7.9.0
- springdoc openapi webflux: 1.2.32 -> 1.8.0
- surefire: 3.2.3 -> 3.5.1 (for e2e tests)
- surefire-ng: 3.2.5 -> 3.5.1 (for e2e tests)

api/src/main/java/io/kafbat/ui/service/KafkaConnectService.java

@@ -1,6 +1,5 @@
 package io.kafbat.ui.service;
 
-import com.fasterxml.jackson.core.type.TypeReference;
 import com.fasterxml.jackson.databind.ObjectMapper;
 import io.kafbat.ui.connect.api.KafkaConnectClientApi;
 import io.kafbat.ui.connect.model.ConnectorStatus;
@@ -31,7 +30,6 @@
 import java.util.stream.Stream;
 import javax.annotation.Nullable;
 import lombok.RequiredArgsConstructor;
-import lombok.SneakyThrows;
 import lombok.extern.slf4j.Slf4j;
 import org.apache.commons.lang3.StringUtils;
 import org.springframework.stereotype.Service;
@@ -107,9 +105,6 @@ public Mono<ConnectorTopics> getConnectorTopics(KafkaCluster cluster, String con
   public Flux<String> getConnectorNames(KafkaCluster cluster, String connectName) {
     return api(cluster, connectName)
         .mono(client -> client.getConnectors(null))
-        // for some reason `getConnectors` method returns the response as a single string
-        .map(List::getFirst)
-        .map(this::parseConnectorsNamesStringToList)
         .flatMapMany(Flux::fromIterable);
   }
 
@@ -118,12 +113,6 @@ public Flux<String> getConnectorNamesWithErrorsSuppress(KafkaCluster cluster, St
     return getConnectorNames(cluster, connectName).onErrorComplete();
   }
 
-  @SneakyThrows
-  private List<String> parseConnectorsNamesStringToList(String json) {
-    return objectMapper.readValue(json, new TypeReference<>() {
-    });
-  }
-
   public Mono<ConnectorDTO> createConnector(KafkaCluster cluster, String connectName,
                                             Mono<NewConnectorDTO> connector) {
     return api(cluster, connectName)