Update Android JDK from 17 LTS to 21 LTS

Why

• Java Development Kit (JDK) is used in Qt for Android (Qt4A) for compiling and running QPA Android Java parts
• Qt4A should have a plan and a policy for JDK updates
• Qt4A should have up-to-date JDK LTS version on place

What

• Qt4A uses Gradle and Android Gradle Plugin (AGP) for building
  • Gradle states the support from Java Virtual Machine (JVM) from 8 to 23, but apparently JDK 17 was required earlier by some Gradle version
  •  
• Qt Quick for Android introduced the Qt Tools for Android Studio for Android Studio based on IntelliJ IDEA
  • Qt Quick for Android has examples with Java & Groovy and Kotlin and DSL
  • Kotlin versions require certain Gradle versions
• JDK has different versions
  • Oracle JDK (from version 11 forward commercial license might be needed.
    • Qt is dual licensed, and thus can not stick only to commercial offering
  • Open JDK
  • Adoptium Eclipse Temurin
  • Amazon Corretto
  • Azul Zulu

How

• Define and communicate a policy to Qt for Android and JDK updates
  • Blog post and development mailing list
• Select a JDK to be used for Qt 6.9 and Qt 6.8  and 6.5 LTS's
• Use the same JDK across Qt and it's tooling incl. Qt Creator (especially Android installation)
• Update the JDK for Qt LTS's versions (if needed)

When

• Qt4A and JDK have LTS and non-LTS versions. Qt LTS life-time is five years (6.8 LTS forward) and JDK "at least three years"
  • Non-LTS versions of JDK has life-time around 6 months
  • Non-LTS versions of Qt has life-time around three patch (X.Y.Z) releases (.Z) 
  • JDK's most recent LTS is 21 and previous is 17 (used in Qt 6.8 LTS atm)
    • JDK 17 was released 9/2021 and "at least three years" was reached by 9/2024
  • Qt 6.8  LTS was released 10/2024 and supported five years
    • Qt 6.5 LTS is supported until 3/2026