Make it clear JDK 11 is required to build without adding `-PjavaVersion=` to arguments of `gradlew` & Prepare to support build using Java 17+

https://github.com/fwcd/kotlin-language-server/blob/bc5309964d24053dd80aed193975629c3e8546ca/gradle.properties#L2

`./gradlew` fails due to this line. Support Java 17. Java 11 will be EOL in this (Oracle JDK's Premier Support) or next year (Temurin).

↓ Java 17 or 20

```
FAILURE: Build failed with an exception.

* What went wrong:
Could not determine the dependencies of null.
> Could not resolve all dependencies for configuration ':buildSrc:buildScriptClasspath'.
> Failed to calculate the value of task ':buildSrc:compileJava' property 'javaCompiler'.
> No matching toolchains found for requested specification: {languageVersion=11, vendor=any, implementation=vendor-specific}.
> No locally installed toolchains match (see https://docs.gradle.org/8.1/userguide/toolchains.html#sec:auto_detection) and toolchain download repositories have not been configured (see https://docs.gradle.org/8.1/userguide/toolchains.html#sub:download_repositories).

* Try:
> Run with --stacktrace option to get the stack trace.
> Run with --info or --debug option to get more log output.
> Run with --scan to get full insights.

* Get more help at https://help.gradle.org

BUILD FAILED in 4s
```

↓ Java 11

```
Starting a Gradle Daemon, 1 stopped Daemon could not be reused, use --status for details

> Task :help

Welcome to Gradle 8.1.

To run a build, run gradlew ...

To see a list of available tasks, run gradlew tasks

To see more detail about a task, run gradlew help --task

To see a list of command-line options, run gradlew --help

For more detail on using Gradle, see https://docs.gradle.org/8.1/userguide/command_line_interface.html

For troubleshooting, visit https://help.gradle.org

BUILD SUCCESSFUL in 1m 52s
11 actionable tasks: 11 executed
<-------------> 0% WAITING
> IDLE
> IDLE
```

Start with gradle.properties at line 2 and reproduce the failure by running ./gradlew with Java 17 or newer, then compare it with the Java 11 run shown in the issue. Update the build configuration so the default command works on Java 17+ while making the Java 11 requirement clear; verify both supported JDK scenarios with ./gradlew.

Written by the indexing model from the issue text.