Add support for Spring Boot 4

[pbartoszek]

I can see following exception when running latest java agent on Spring Boot 4 running java 25.

java.lang.NoSuchMethodError: 'boolean org.springframework.http.HttpHeaders.containsKey(java.lang.Object)'
	at co.elastic.apm.agent.resttemplate.SpringRestRequestHeaderSetter.setHeader(SpringRestRequestHeaderSetter.java:30)````

[fbaligand]

I am very interested by this enhancement!
Currently, I can't use Elastic APM agent on my Spring Boot 4 applications, that is really an issue for my application and my company.
And some other applications do not migrate to Spring Boot 4, until Elastic APM agent is compatible.

[jackshirazi]

This should be supported by our EDOT agent

[pbartoszek]

@jackshirazi I just found on https://www.elastic.co/docs/solutions/observability/get-started/other-tutorials/tutorial-monitor-java-application info

We recommend using the [Elastic Distribution of OpenTelemetry (EDOT) Java SDK](https://www.elastic.co/docs/reference/opentelemetry/edot-sdks/java/setup) as the
preferred way to instrument your Java application. The EDOT Java SDK provides OpenTelemetry-based instrumentation and can send telemetry data to Elastic.

is this the case that for new integrations EDOT is preferred choice?