README: use McpClientCustomizer in the client customizer snippets

[shin1488]

The @Bean snippets in the README and in AuthenticationMcpTransportContextProvider's javadoc declare the bean as McpSyncClientCustomizer. Spring AI removed that type in 2.0.0-M3, replacing the sync and async client customizers with the parameterized McpClientCustomizer<B>. This project moved to M3 in d477aa3, which migrated the Java sources but not the documentation, and now targets 2.0.0 — so the snippets no longer compile against the version it ships with.

Configurations of this bean in the repository already use the current type — for example samples/sample-mcp-client-webclient/.../McpConfiguration.java, samples/integration-tests/src/main/.../McpClientConfiguration.java and mcp-client-security-spring-boot/.../McpOAuth2ClientConfigurations.java:

@Bean
McpClientCustomizer<McpClient.SyncSpec> syncClientCustomizer() {
    return (name, syncSpec) -> syncSpec.transportContextProvider(new AuthenticationMcpTransportContextProvider());
}

The lambda is unchanged — McpClientCustomizer takes the same (name, spec) arguments — so this is the type only. Four snippets in the README and the one in the javadoc are updated, along with the sentence naming McpSyncClientCustomizer and McpAsyncClientCustomizer, which now names McpClientCustomizer for both. In the javadoc the angle brackets are written as HTML entities, as the surrounding <pre> block already does for the annotation.