)

## Problem

`./run-example -Dssl discard-client` against `./run-example -Dssl
discard-server` does not complete the TLS handshake — it hangs until the
handshake times out.

## Root Cause

`DiscardClient.main` builds its `SslContext` via
`ServerUtil.buildSslContext()`:

```java
// example/src/main/java/io/netty/example/util/ServerUtil.java
public static SslContext buildSslContext() throws ... {
    if (!SSL) return null;
    SelfSignedCertificate ssc = new SelfSignedCertificate();
    return SslContextBuilder.forServer(ssc.certificate(), ssc.privateKey()).build();
}
```

`SslContextBuilder.forServer(...)` produces a context whose `isClient()`
is `false`, so its `SSLEngine` is created with `setUseClientMode(false)`
(see `JdkSslContext`). When the example's "client" attaches that engine
to its `SslHandler`, both ends of the connection are configured as TLS
servers: each side waits for an inbound `ClientHello` from the other,
and neither sends one. The handshake deadlocks until the configured
handshake timeout fires.

## Fix

In `DiscardClient`, build a client `SslContext` with
`InsecureTrustManagerFactory` when `-Dssl` is set, and leave `sslCtx` as
`null` otherwise. This is the same pattern already used by
`ObjectEchoClient`, `HttpUploadClient`, `SecureChatClient`,
`Http2Client`, `MemcacheClient`, and other client examples in the same
module.

The server side (`DiscardServer`) is unchanged and continues to use
`ServerUtil.buildSslContext()` correctly.

## Tests Added

The `example` module has no unit-test infrastructure (`example/src/`
contains only `main`); examples are runnable demos rather than testable
units. The fix is verified by inspection against the matching `if (SSL)
{ SslContextBuilder.forClient()... }` pattern in the other client
examples cited above.

## Impact

- `./run-example -Dssl discard-client` against `./run-example -Dssl
discard-server` now completes the TLS handshake instead of timing out.
- Plaintext mode (no `-Dssl`) is unchanged: `sslCtx` is `null` exactly
as before.
- Only `DiscardClient.java` is touched. `DiscardServer.java`,
`DiscardClientHandler.java`, and `DiscardServerHandler.java` are
unchanged.

Other example clients (`EchoClient`, `FactorialClient`,
`WorldClockClient`, `TelnetClient`) call the same
`ServerUtil.buildSslContext()` and exhibit the same hang under `-Dssl`.
Those are kept out of this PR to keep the scope tied to the reported
issue and can be addressed in a follow-up.

Fixes #14499

(cherry picked from commit b55fabc)