Drop Java 8 (JRE 1.8) support

[szysas]

Some reasons for moving project from Java 8 to Java 17 as minimum.

• Modernize Codebase:
  • Take advantage of language enhancements and new APIs introduced in Java 9 through Java 17 to keep the open-coap project modern and efficient
• Some Java 8 API became deprecated in 21
• Compiler Warnings:
  • warning: [options] source value 8 is obsolete and will be removed in a future release
  • warning: [options] target value 8 is obsolete and will be removed in a future release
• Library Compatibility:
  • Some libraries, like logback-classic and mockito-core, may not support Java 8 and it prevents using the latest versions.
• Gradle Plugin Compatibility:
  • Some Gradle plugins may not work with Java 8 - workaround is to use the toolchain feature in Gradle to specify the Java version for compiling and running the project.

References:

• https://www.oracle.com/java/technologies/java-se-support-roadmap.html
• https://adoptium.net/support/