Make SslProvider.JDK the default provider for http2 example.

Motivation:
When running the http2 example no SslProvider is specified when calling
SslContext.newServerContext. This may lead to the provider being
determined depending on the availabilty of OpenSsl. But as far as I can
tell the OpenSslServerContext does not support APLN, which is the
protocol configured in the example.
This produces the following error when running the example:

Exception in thread "main" java.lang.UnsupportedOperationException:
OpenSSL provider does not support ALPN protocol
io.netty.handler.ssl.OpenSslServerContext.toNegotiator(OpenSslServerContext.java:391)
io.netty.handler.ssl.OpenSslServerContext.<init>(OpenSslServerContext.java:117)
io.netty.handler.ssl.SslContext.newServerContext(SslContext.java:238)
io.netty.handler.ssl.SslContext.newServerContext(SslContext.java:184)
io.netty.handler.ssl.SslContext.newServerContext(SslContext.java:124)
io.netty.example.http2.server.Http2Server.main(Http2Server.java:51)

Modifications:
Force SslProvider.JDK when creating the SslContext since the
example is using APLN.

Result:
There is no longer an error if OpenSsl is supported on the platform in
use.