The logic for the cache in `JwtRealm` produces a broken `Cache` instance to some degree. 
We create a `BytesArray` from the user credential hash:

```java
            final BytesArray jwtCacheKey = isCacheEnabled() ? new BytesArray(jwtAuthenticationToken.getUserCredentialsHash()) : null;
            if (jwtCacheKey != null) {
                final User cachedUser = tryAuthenticateWithCache(tokenPrincipal, jwtCacheKey);
```

and use it as as a cache key in a `Cache` instance down the line.
But then we also clear the bytes in that `BytesArray` to `0` in org.elasticsearch.xpack.security.authc.jwt.JwtAuthenticationToken#clearCredentials. 

This leads to the hash map in the `Cache` getting filled with loads of `BytesArray` that all have the same 0-filled content but that were not equal to each other when first inserted into the map, causing the map to put them all in the same bucket.

<img width="1233" alt="image" src="https://github.com/elastic/elasticsearch/assets/6490959/86a914d7-a2d4-4b01-ad99-e0d14e71a58b">

ref path to the broken map:

<img width="1436" alt="image" src="https://github.com/elastic/elasticsearch/assets/6490959/87ac6f78-bc09-41c4-9a79-28fbbd14f6e6">

x-pack/plugin/security/src/test/java/org/elasticsearch/xpack/security/authc/jwt/JwtRealmTestCase.java
if (jwtIssuerAndRealms != null) {
            jwtIssuerAndRealms.forEach(jwtIssuerAndRealm -> {
                jwtIssuerAndRealm.realm.close(); 
                jwtIssuerAndRealm.issuer.close(); 
            });
        }
        resourceWatcherService.close();
        terminate(threadPool);
    }
    protected void verifyAuthenticateFailureHelper(
        final JwtIssuerAndRealm jwtIssuerAndRealm,
        final SecureString jwt,
        final SecureString clientSecret
    ) throws InterruptedException, ExecutionException {
        final ThreadContext tc = createThreadContext(jwt, clientSecret);
        final JwtAuthenticationToken token = (JwtAuthenticationToken) jwtIssuerAndRealm.realm.token(tc);
        final PlainActionFuture<AuthenticationResult<User>> plainActionFuture = PlainActionFuture.newFuture();
        jwtIssuerAndRealm.realm.authenticate(token, plainActionFuture);
        assertThat(plainActionFuture.get(), notNullValue());
        assertThat(plainActionFuture.get().isAuthenticated(), is(false));
    }
    protected List<JwtIssuerAndRealm> generateJwtIssuerRealmPairs(
        final int realmsCount,
        final int authzCount,
        final int algsCount,
        final int audiencesCount,
        final int usersCount,
        final int rolesCount,
        final int jwtCacheSize,
        final boolean createHttpsServer