make compatible with Kotlin 2.1.0

[XilinJia]

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: 4 failed tests related to BacklinksTests XilinJia/realm-kotlin#1 (comment).

[cla-bot]

Realm welcomes all contributions! The only requirement we have is that, like many other projects, we need to have a Contributor License Agreement (CLA) in place before we can accept any external code. Our own CLA is a modified version of the Apache Software Foundation’s CLA. Our records show that CLA has not been signed by @XilinJia. Please submit your CLA electronically using our Google form so we can accept your submissions. After signing the CLA you can recheck this PR with a @cla-bot check comment. The GitHub usernames you file there will need to match that of your Pull Requests. If you have any questions or cannot file the CLA electronically, make a comment here and we will be happy to help you out.

[XilinJia]

is CLA still needed for the community branch?

[MrJohn10]

@clementetb Any updates?

[cla-bot]

Realm welcomes all contributions! The only requirement we have is that, like many other projects, we need to have a Contributor License Agreement (CLA) in place before we can accept any external code. Our own CLA is a modified version of the Apache Software Foundation’s CLA. Our records show that CLA has not been signed by @XilinJia. Please submit your CLA electronically using our Google form so we can accept your submissions. After signing the CLA you can recheck this PR with a @cla-bot check comment. The GitHub usernames you file there will need to match that of your Pull Requests. If you have any questions or cannot file the CLA electronically, make a comment here and we will be happy to help you out.