make compatible with Kotlin 2.1.0

There are a few changes in this PR: 1. added a build script for Linux 2. updated related Kotlin code to work with Kotlin 2.1.0 3. changed project structure to make Intellij IDE work, buildSrc is moved out of scope io.realm, gradlew now should be run in the root directory rather than in packages 4. amended Contributing.md to reflect the structure changes 5. updated most dependencies Facts: 1. I've only tested the project on Linux but kept the project open for over two weeks to let others test it in various conditions. 2. as it stand, testing can be run on dev versions not on maven artifacts. 3. tests run resulting 4 failures with Kotlin 2.1.0 that also occurred with Kotlin 2.0.x before any modifications were made. documented here: https://github.com/XilinJia/realm-kotlin/issues/1#issue-2812643527.