Duplicate Spring Boot 3 files for Spring Boot 4

[adinauer]

I tried duplicating the springBoot3 files to make opentelemetry-spring-boot-starter work with Spring Boot 4.

While I was able to deploy a modified version to my local Maven repository and confirm it fixes the ClassNotFoundException for org.springframework.boot.autoconfigure.web.client.RestClientAutoConfiguration, I was unable to make it coexist with Spring Boot 3 configuration.

IIRC Spring Boot 2 used spring.factories and support for org.springframework.boot.autoconfigure.AutoConfiguration.imports was added in Spring Boot 3, so it was fine for those to coexist. For Spring Boot 4 however, there'd have to be a mechanism of configuring whether to use imports from the Spring Boot 3 vs 4 sources. Event with having separate resources directories, I wasn't able to make this work since they still end up in the same JAR.

I might be wrong here^, if so, we can continue on this approach. If this holds true, I recommend duplicating the spring-boot-autoconfigure and spring-boot-starter modules for Spring Boot 4. I'll open another PR for this shortly. I opened this PR more for documenting what I tried, but recommend closing it in favor of #14416.

Attempt at fixing #14363

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