chore: Use Java 21 for building JMeter

[vlsi]

Description

Java 21 has been released some time ago, so we can use it as the default build JDK.

Motivation and Context

We want to use the recent javac to avoid bugs in the old compilers. Java 21 supports --release 8, so we would still support running with Java 8.

TODO

• Wait for Gradle 8.5 release. It would enable us to target Java 21 in the build scripts. Otherwise, we would need both Java 17 (for the build scripts) and Java 21 for the main build
• Deal with "Java 21 not being released by Semeru". Currently, we expect that "default build Java" is to be available in all the distributions (see https://github.com/apache/jmeter/blob/3c6cacb507cddfe4c337213f364f2b5730d6b5d1/.github/workflows/main.yml#L58-L62 ), however, it is failing for Semeru