Fix ordering of Kotlinx Serialization CodecCustomizer

[dmitrysulman]

Kotlinx Serialization CodecCustomizer introduced in d161aaf lacks an explicit order, meaning it defaults to LOWEST_PRECEDENCE. This prevents user-provided customizers from overriding Spring Boot defaults, as the default customizer runs last and overwrites previous configurations.

A similar issue for message converters was fixed in 3948e7c. The Jackson CodecCustomizer is already explicitly ordered:

spring-boot/module/spring-boot-http-codec/src/main/java/org/springframework/boot/http/codec/autoconfigure/CodecsAutoConfiguration.java

Lines 68 to 77 in 21a3eec

	@Bean
	@Order(0)
	@ConditionalOnBean(JsonMapper.class)
	CodecCustomizer jacksonCodecCustomizer(JsonMapper jsonMapper) {
	return (configurer) -> {
	CodecConfigurer.DefaultCodecs defaults = configurer.defaultCodecs();
	defaults.jacksonJsonDecoder(new JacksonJsonDecoder(jsonMapper));
	defaults.jacksonJsonEncoder(new JacksonJsonEncoder(jsonMapper));
	};
	}

This PR adds @Order(0) to the kotlinxJsonCodecCustomizer bean to address this issue.

See #48070 and #48609

@bclozel fyi