### Elasticsearch Version

main

### Installed Plugins

_No response_

### Java Version

_bundled_

### OS Version

MacOS

### Problem Description

When there are 2 or more concurrent `_update` requests issued, one of them may fail with `ResourceAlreadyExistsException` exception which is misleading.


### Steps to Reproduce

1. Spawn a number of `_update` requests at the same time
2. Look at the results of these requests
3. Observe `ResourceAlreadyExistsException` being thrown in the case of conflict

### Logs (if relevant)

_No response_
x-pack/plugin/transform/qa/single-node-tests/src/javaRestTest/java/org/elasticsearch/xpack/transform/integration/TransformUpdateIT.java
try {
            if (useSecondaryAuthHeaders) {
                startAndWaitForTransform(
                    transformId,
                    transformDest,
                    BASIC_AUTH_VALUE_TRANSFORM_ADMIN_NO_DATA,
                    BASIC_AUTH_VALUE_TRANSFORM_ADMIN_1,
                    new String[0]
                );
            } else {
                startAndWaitForTransform(transformId, transformDest, BASIC_AUTH_VALUE_TRANSFORM_ADMIN_1);
            }
            fail("request should have failed");
        } catch (ResponseException e) {
            assertThat(e.getResponse().getStatusLine().getStatusCode(), equalTo(500));
        }
        assertBusy(() -> {
            Map<?, ?> transformStatsAsMap = getTransformStateAndStats(transformId);
            assertThat(XContentMapValues.extractValue("stats.documents_indexed", transformStatsAsMap), equalTo(0));
        }, 3, TimeUnit.SECONDS);
        
        final Request updateRequest = useSecondaryAuthHeaders
            ? createRequestWithSecondaryAuth(
                "POST",
                getTransformEndpoint() + transformIdCloned + "/_update",
                BASIC_AUTH_VALUE_TRANSFORM_ADMIN_NO_DATA,
                BASIC_AUTH_VALUE_TRANSFORM_ADMIN_1
            )
            : createRequestWithAuth("POST", getTransformEndpoint() + transformIdCloned + "/_update", BASIC_AUTH_VALUE_TRANSFORM_ADMIN_1);
        updateRequest.setJsonEntity("{}");