Remove Kotlin plugin entirely from the root project

[Pankraz76]

It seems the plugin was not applied right from the beginning making me winder if its because of ht issue being linked:
https://youtrack.jetbrains.com/issue/KT-30276

As its resolved it might be working not, making the workaround obsolete.

Thanks for considering.

[Pankraz76]

somehow main is not working as well no my system, giving me this error upfront:

> Task :build-plugin:spring-boot-gradle-plugin:antora
(node:4582) [DEP0180] DeprecationWarning: fs.Stats constructor is deprecated.
(Use `node --trace-deprecation ...` to show where the warning was created)

> Task :build-plugin:spring-boot-maven-plugin:antora
(node:4652) [DEP0180] DeprecationWarning: fs.Stats constructor is deprecated.
(Use `node --trace-deprecation ...` to show where the warning was created)

> Task :core:spring-boot:compileJava
Note: Processing Log4j annotations
Note: Annotations processed
Note: Processing Log4j annotations
Note: No elements to process
/Users/vincent.potucek/IdeaProjects/spring-boot/core/spring-boot/src/main/java/org/springframework/boot/context/properties/bind/ValueObjectBinder.java:423: error: [NullAway] Method returns @Nullable String @Nullable [], but overridden method returns String [], which has mismatched type parameter nullability
                public @Nullable String @Nullable [] getParameterNames(Constructor<?> constructor) {
                                                     ^
    (see http://t.uber.com/nullaway )
/Users/vincent.potucek/IdeaProjects/spring-boot/core/spring-boot/src/main/java/org/springframework/boot/logging/structured/StructuredLogFormatterFactory.java:240: error: [NullAway] referenced method returns @Nullable, but functional interface method java.util.function.Function.apply(T) returns @NonNull
                                        ArgumentResolver.from(StructuredLogFormatterFactory.this.instantiator::getArg));
                                                              ^
    (see http://t.uber.com/nullaway )

> Task :buildpack:spring-boot-buildpack-platform:dockerTest FAILED

DockerApiIntegrationTests > pullImage() FAILED
    org.springframework.boot.buildpack.platform.docker.transport.DockerEngineException at DockerApiIntegrationTests.java:39

1 test completed, 1 failed

> Task :core:spring-boot:compileJava FAILED
2 errors

Found test failures in 1 test task:

:buildpack:spring-boot-buildpack-platform:dockerTest
    org.springframework.boot.buildpack.platform.docker.DockerApiIntegrationTests > pullImage()

[Incubating] Problems report is available at: file:///Users/vincent.potucek/IdeaProjects/spring-boot/build/reports/problems/problems-report.html

FAILURE: Build completed with 2 failures.

1: Task failed with an exception.
-----------
* What went wrong:
Execution failed for task ':buildpack:spring-boot-buildpack-platform:dockerTest'.
> There were failing tests. See the report at: file:///Users/vincent.potucek/IdeaProjects/spring-boot/buildpack/spring-boot-buildpack-platform/build/reports/tests/dockerTest/index.html
==============================================================================

2: Task failed with an exception.
-----------
* What went wrong:
Execution failed for task ':core:spring-boot:compileJava'.
> Compilation failed; see the compiler output below.
==============================================================================

Deprecated Gradle features were used in this build, making it incompatible with Gradle 9.0.

You can use '--warning-mode all' to show the individual deprecation warnings and determine if they come from your own scripts or plugins.

For more on this, please refer to https://docs.gradle.org/8.14.3/userguide/command_line_interface.html#sec:command_line_warnings in the Gradle documentation.

BUILD FAILED in 34s
370 actionable tasks: 67 executed, 303 up-to-date
➜  spring-boot git:(fix-resolved-issue) 

[snicoll]

You need Java 24 to build main.

[snicoll]

The build fails for me and those 4 tests don't extend from base as far as I can see so I am not sure what this PR is trying to do. If they were, there are more kotlin tests that'd be affected.